Muscles need a stable foundation, a fixed endpoint.
Whether you are trying to activate a muscle or relax it, it can help to anchor one end of the muscle.
What does this mean?
Options for Anchoring the Hamstrings
Your hamstrings attach to the back of the fibula and tibia (the two lower leg bones). They also attach to the ischial tuberosity (the sitting bone which is at the bottom back corner of the hip bone.) And if you consider the short head biceps femoris to be a hamstring, then they also attach to the femur.
If you want to control your hamstrings then you either stabilize or anchor the lower leg bones. Or you stabilize or anchor the hip bone. An in the case of the short head biceps femoris, then you could also stabilize the femur.
But wait. The hamstring tendons interlock with the tendons of the gastrocnemius. So another way to anchor the hamstrings is to activate the gastrocnemius.
Note that you could also anchor the hamstring by creating a downward pull on the same side ASICs.
How to Anchor the Hip Bone
As for anchoring the hip bone, you could anchor it by stabilizing the hip joint. You then unify the mass of the hip bone and femur which then may be more massive in comparison to the lower leg bones (assuming that they in turn aren't stabilized relative to each other and/or the bones of the ankle and foot.)
You could also anchor the hip bone by stabilizing the S.I. joint, lumbar spine and ribcage, or even by stabilizing both S.I. joints. In the first case you augment its affective mass by tying it to the lumbar spine and ribcage. In the other you do so by tying it to the sacrum and opposite hip joint.
Note that in some cases you can anchor muscles by activating muscles in the same connective tissue train.
Muscles need to be at an effective length in order to activate effectively (or to activate at all.)
If a muscle is too short or too long then it can't effectively activate or relax.
Part of muscle control training can include improving this range so that muscles can activate over a longer range.
There are two ways to move a muscle into its range of effectiveness.
The most basic is to change the position of the joint (or joints) that it works on. The hamstrings can be difficult to activate with the knee closed (flexed). They become easier to activate when the knee is straight or less bent.
Some muscles lie over other muscles. The rectus femoris lies over the vastus intermedius. The IT band (which is worked on by the tensor fascia latae and the superior fibers of the gluteus maximus) lies over the vastus lateralis muscle. The superior fibers of the gluteus maximus lie over the deep fibers. The sartoris and gracilis pass over some combination of the adductors and the vastus medialis.
A way to add length to all of these muscles is to activate the underlying vastus or adductor muscles.
This could be thought of as the compliment to stability or an offshoot of stability.
When you have a fixed endpoint you can then clearly define the direction you want to move in with respect to that endpoint, whether towards the fixed endpoint or away from it.
You could also focus on not moving relative to the fixed point.
Muscles need an opposing force for sustained activation.
Muscles generate a force when activated. To stay activate they need an opposing force to work against.
This force can be provided externally and or via body weight. But in the absence of either of these, then opposition can be provided internally via opposing muscles.
Curling a dumbell, if you hold the weight mid curl then your biceps is going to stay engaged because it is working against the weight of the dumbell. Flexing your biceps without any external weight, then the force it works against is the triceps.
Based on the previous point, if muscles need a force to work against in order for sustained activation then if a muscle is active it indicates that it is working against an opposing force. That can be an opposing muscle, body weight, or some external force.
Internal proprioception requires two types of input, an active input provided directly by muscle activation and a passive one provided by connective tissue tension or tension induced deformation (which equals stretching.)
The passive system doesn't work without the active system.
You need muscle control both for movement and to sense your body.
You also need it for joint lubrication.
Muscle tension affects both ligaments and tendons. For example, the sacrotuberous ligament which connects the sacrum to the ischial tuberosity is affected by tension from the hamstrings, piriformis and the gluteus maximus muscle. Thus tension from these muscles can affect the SI joint via this ligament.
Other ligaments can be affected by muscle tension via intermediary joint bursars, in particular the knee joints.
Understanding that both ligaments and tendons can be affected by muscle activation leads to the idea that joint capsule tension can be affected by muscle tension which is important when considering joint lubrication.
There are three types of joint lubrication: Boundary Layer, Hydrodynamic and Hydrostatic.
The latter two prevent articulating surfaces from rubbing, reducing wear and tear.
With these latter two lubrication methods, joint surfaces can adjust relative to each other which means that tension in the joint capsule can be redistributed reducing the risk of joint capsule tearing.
Both require muscle activation to work.
Boundary Layer Lubrication
Boundary layer lubrication occurs when joint surfaces rub against each other.
This could be considered the default lubrication mechanism under low loads. Because we are biological, this system can replenish itself.
Hydrodynamic lubrication occurs when there are large speed differences between joint surfaces. The mechanism is the same that drives hydroplaning.
Above a particular speed a wedge of water is driven between the wheels of the car and the road surface that causes the car to glide. Scientists trying to create longer lasting artificial joints have found that this seems to be the case for our joints also.
This could occur in rapid cadence running or walking, perhaps as the lower leg returns from a back swing and swings forwards just as the femur's forward movement is halted prior to it moving rearwards.
Hydrostatic lubrication occurs as a result of muscle tension increasing joint capsule tension which in turn pressurized joint fluid so that it keeps mating surfaces from directly rubbing.
Since joints are critical or key structures, the brain may limit muscle activation where it thinks a joint may be damaged.
Muscles move joints or stabilize them. However muscles may also be "controlled" or limited by joints to keep joints protected.
To better understand the interaction of joints and muscles it can help to understand that muscles have overlap.
One of the main reasons for this is so that we can move as freely as we do. As one muscle of set of muscles move out of effective range, another set takes over (or augments). This overlap also means that muscles can substitute for each other given problems. Substitution can be governed by the need to keep joints protected.
Another reason for overlap is to deal with differences in load.
Standing on both legs, both knees can share the load of the body. Standing on one leg, then that knee has to bear the weight of the whole body.
Knee capsule tension has to be increased in order to maintain clearance between joint surfaces. Add more weight, say a barbell, then tension has to be increased further. This can come from other muscles, multijoint muscles, also activating.
So the more weight a joint is dealing with, the more of the body that has to act together. And that may be one reason for getting stuck at a particular weight. You aren't able to use your muscles in such a way to keep your joints protected when dealing with the heavier weight.