Learn revolved triangle (Parivrtta Trikonasana) properly: square the pelvis, hand on a block, twist with a long spine. What studies show about twisting poses.
How do you get revolved triangle without the pelvis tipping away? The honest answer: with a block, a short stance and the goal of taking the twist from your spine. Parivrtta Trikonasana ("revolved triangle") is a standing twist over the front leg, in which the hand opposite the front leg supports on the outside of the foot or on a block and the other arm points to the ceiling. It combines a forward bend and a twist in one pose.
That makes it one of the poses in which your body honestly shows what does not cooperate: the hamstrings when folding, the hip when keeping it square, the thoracic spine when twisting. That is practice experience. If you learn the pose with a block and a wide stance, you mainly practice alignment, and that pays off in almost every later twist.
I found no study on the pose itself. But on a related one: in a study of 34 women, researchers compared an upright twist with a "semi-triangle twist", a twist in deep hip flexion. The second variation required more trunk rotation (44 to 70 degrees) and higher activity of the back extensors [1]. That is transfer; whether this twist matches revolved triangle is not clear-cut, but the direction is clear: twisting in a fold challenges the back especially. And: experienced practitioners looked the same as beginners from the outside but tended to use their rectus abdominis more specifically on the inside [1].
Stand in a short stance, about one foot length back. The back foot turns out 45 degrees, the heel stays on the floor. Both hips face forward. A wider track (feet further apart sideways) makes the pose more stable.
Fold forward with a long back, hands on hips or on blocks. The legs are active, the front knee soft.
Milestone: You hold 5 breaths in this forward bend without the hip tipping sideways.
Place the hand opposite the front leg on the block, outside the front foot. Turn the torso open, the other arm goes to the ceiling. The twist comes from the chest, not from the pelvis. The hips stay square, even if the elbow therefore goes less far.
Stay five to eight calm breaths. The gaze stays down or goes to the side, later to the ceiling. To release, turn back, rise with a long back and switch sides.
Milestone to move on: You hold the pose for 5 breaths per side, hips square, back long, hand on block. Then you can swap the block for the floor.
In yoga, the trunk is the most frequently injured region: in US emergency department data, 46.6% of injuries involved the trunk, and sprains and strains were the most common diagnosis [2]. The literature also names lower back and hamstrings as problem areas [3]. Twists in a fold put pressure on the spine, so: length first, then rotation, never jerky. Four weeks of daily stretching raised tolerance by 8 degrees in one study, but not muscle length [4]: do not be misled by the feeling that "it goes further". With tingling, numbness or radiating pain, release. This guide explains and motivates; it does not replace supervised training.
Set your feet wider than you think. Place the front foot more to the side than in the side stretch, roughly in line with the other foot. This is a practice trick: a wider track makes balance much easier and the hips stay square more easily. If you can square the pelvis, you also have more room for the twist.
Far enough that the hips stay square and the back stays long. There is no number for that. In the study of a similar twist, trunk rotation was 44 to 70 degrees [1], but that is not a target for you.
Because the stance is too narrow and the pelvis tips along. Take the feet further apart (wider track) and a hand on a block. Then the pose becomes much more stable.
In yoga, the trunk is the most frequently affected region [2]. With a block, a long back and slow twisting, the pose is quite doable for healthy people. With back pain or disc problems, clarify it beforehand.
The hand opposite the front leg supports, best on a block outside the front foot. The other arm points to the ceiling. If that compresses the back too much, put the upper hand on the hip and twist only as far as the hips stay square.
No. Triangle and side stretch come first because they prepare the hamstrings and alignment. Only when both sit securely do you twist.
Half Lotus (Ardha Padmasana): Open the Hip, Never Force the Knee