I used to hate revolved triangle. It's difficult. But, it's a great pose for strengthening the legs, the back and the waist. It can even be used to strengthen the arms.
Generally the goal in revolved triangle can be to get the hand to the floor (if you are less flexible) and then to get the bottom hand to the floor outside the foot (if you are moderately flexible but want to be more flexible). Using revolved triangle as a yoga pose for strengthening the body, neither of these goals really matters. What matters is that you can control your torso without even touching your hand to the floor.
With the torso horizontal (or nearly so) you can use the pose to strengthen the legs by lifting the bottom hand. And you can use it to strengthen the hips and waist and intercostals by using those same muscles to both support the torso and twist it.
The following instructions are to help you learn your body at the same time as you work at strengthening it.
In triangle forward bend (as a prep for revolved triangle) one of the actions that you can work on to help give your twisting muscles a foundation from which to turn the ribcage is to push your pelvis rearwards.
Pushing the pelvis rearwards.
So that you get used to the feeling of pushing your pelvis rearwards, first relax the front leg with hands on the floor to help support your body. If your hands don't reach the floor then bend the front knee or rest your hands on your shin, or on yoga blocks.
If using yoga blocks, position them below your shoulders and adjust the height so that you can comfortably rest your hands on them and support the weight of your upper body.
Then slowly push your foot forwards and your pelvis back. Reach your ribs forwards and as you do so lift your hands by bending your elbows. Pause, then support your body with your hands again and relax. Repeat a few times.
Using the legs to push (or pull) the pelvis rearwards, you can then reach your ribs forwards away from the pelvis. Initially focus on the fronts of your ribs, then the sides, then also the backs of your ribs. Try to create space at all four sides of your ribcage.
Reaching ribs away from pelvis.
One reason for creating space is that it gives the ribs room to move relative to each other. Another reason is that it adds tension to the intercostal muscles, the muscles between the ribs, making it easier to use them to help deepen the twist in revolved triangle.
Adding a twist to create revolved triangle, you can do the same thing, push your hips back while reaching your ribs forwards (away from your pelvis). Then, reach the front-leg hand upwards, turning your ribcage towards the front-leg-side.
Lengthening spine then twisting.
Starting with a relaxed twist, and using a yoga block for your bottom hand if need be, push your hips back, then reach your ribs forwards, and as you reach your ribs forwards try to deepen the twist. Then relax keeping the twist as much as possible. Then repeat but try to deepen the twist, going a little bit deeper each time.
So that you can go deeper you can focus on either the front, sides or back of your ribcage. Try to create space where ever you find tightness so that you have room to deepen the twist.
One option is to lift the hand in revolved triangle as you deepen the twist so that you use only the muscles of your waist and ribcage to drive the twist.
Another option is to use the bottom hand against your shin to turn your ribcage. Here a pattern you can use is to use your hand to drive the twist while inhaling, then try to keep the twist as you relax the hand while exhaling.
Using the arm to help the twist.
When using your hand, rather than just using brute strength, continue to feel your ribs (and spine). Work at creating space (or "feeling") in your ribcage so that you are using your arm and the muscles of the torso together to generate the twist. Imagine using your arm to assist the muscles of your torso. Then each time you relax your arm see if you can use just the muscles of your waist and ribcage to hold the twist with the idea that eventually you can use those muscles to enter the twist without using your arm to help.
Should you be keeping your pelvis level in revolved triangle?
It is one way of doing the pose. It teaches you to feel when your pelvis is level, which is good for awareness. But once you can feel that your pelvis is level, you can vary the position to increase awareness and flexibility.
Two options are to tilt your pelvis into the twist or away from it.
Another option is to push the hips to the front-leg side or the back leg side.
All of these options give the pose a different feel. They get the muscles used to working in different ways. Some of these positions are uncomfortable and so rather than just holding them, work in and out of them smoothly, with your breath.
So lets say you still want to get your hand to the floor outside the foot in revolved triangle, how do you go about doing it?
To get your hand outside the foot in revolved triangle, one option is to start from a forward bend and side bend your spine so that your back leg hand is outside the foot with shoulder over the wrist. From there, keeping the shoulder in place over the hand, twist.
Side bending the spine (so hand it outside foot) then twisting.
Another option is to do the equivalent of a side bend after twisting, so that the side bending action carries the hand and shoulder to the outside of the foot. From there you could try to open up the front of the uppermost side of the ribcage so that the chest is at least partially open.
Another option is to turn the pelvis to bring the body over the outside of the foot. With right foot forwards this would mean pulling the right hip back and/or moving the left hip forwards. You could do this with the intent of keeping the pelvis level, or you could work towards doing it while keeping the pelvis level. Explore the options and notice how each one feels. Or explore the options and try to make each option feel as comfortable as possible.