If in isolation you flatten the ski, releasing the edge and nothing else, the skis will drift away, the CoM will not crossover and there will be no crossunder either. Releasing the ski causes the turn to widen, the skis to drift away and to lose centripetal forces. Something else has to be executed to create a transition besides merely releasing the edges. The CoM itself has to be released! If you release the BoS in isolation the CoM will become out of balance to the inside which actually would constrain crossover even more.
with exam passing wedge Christies the uphill ski cannot stem up the hill. So the skier has to somehow steer the uphill ski without pushing the tail out; while not steering the downhill ski until later.
The way I perform these is to relax my downhill leg, allowing uphill leg to support my weight. That instantly puts me crossed over slightly. There is no need to move the CoM up the hill nor down the hill. Simply relax the downhill leg a bit and that instantly changes the BoS in such a way that the CoM is now on the downhill side of it slightly. This, combined with tipping the now pressured uphill ski, will cause it to engage and turn down the hill without a heel push. at about the same time you do this relaxation of the downhill leg, you can flatten it. Releasing that ski is still relevant but releasing the ski edge angle alone does not release the CoM. The CoM is released by relaxing the leg, or for many of you but NOT me, the CoM can be pushed across by the uphill leg extension and avoid much relaxation of the downhill leg