I've recently had a couple of teachers adjust my utthita hasta padangustasana giving me the same instruction, to relax the hip flexors in the first two stages (front and out to side) and only engage when you release the leg to hold it in place.
So previously I've worked out that I have to drop the sitting bone / drop the ball into the socket or however you want to describe it. This has been working just fine, hips beginning to open slowly and being able to hold the posture for longer without falling out.
But this has transformed the pose to even greater extents! No way? - I hear you cry! Yes way. SO this is how I think it works...
In the first two stages you relax the hip, drop the sitting bone to level the hips to get your balance, you then use and engage your arm, your biceps, to hold the leg up, pulling the leg up as if it were a 'dead weight'. Still leading with your arm only you take the leg out to the side, letting the leg follow, still relaxing the hip flexors, you continue to hold the leg, using the bicep to pull the leg towards the body. You then, still using the arm only, take the leg back out to the front - in fact it's not the front, I'd say hold it about 10 degrees out from the midline of the body - you 'bow' to the leg pulling it to its highest point. You stand straight and release said leg and then...only THEN...engage all the hip flexors in the world to hold the leg up!!
Activate where necessary, relax where possible. It appears that the passive stretch of the hip in this posture in the first two stages stretches the required muscles to get a deep posture, without wearing them out, leaving the strength of the hip flexors for holding the leg in the final stage. You will find the posture will not wear you out, you will feel the whole posture lighter and the passive stretch of the hip flexors will allow you to hold the leg higher in the final stage.