Hmm.. Ari, Interesting question/thoughts...I suppose I've been lucky because this movement is not difficult for me..aughh for once I must have done something right! as far as alighment goes that is
LOL I'm not really an expert though because in my classes the grand plies have not always been an integral part of class..not used EVERY time like in ballet. I only had one teacher last semester 2 x a week who liked to use them..of course the turned out grand in second is the best as far as lack of pressure but I also think that you are working so hard in the center, parallel grand that there is really no room for sitting as there is at the ballet barre. You are much more aware of using the muscles in this movement when it is executed in the center and you don't have anything to hang on to..it doesn't leave the time to relax and sag or drop your bottom or pause..
you get down there in a controlled way and then you are ready to come back up!
Sorry I know this answer isn't exactly what you're looking for but this has been my experience and again..I'm not an expert..although I run my mouth a lot!
<P>------------------<BR><BR>"If it's self expression you are looking for the place for you is the analyst's couch" - Merce Cunningham