This was a video taken of my 8 year old WP gelding on his second day of learning lead changes (don't mind his first two attempts, he was fresh into his lesson). He wouldn't be able to switch so smoothly if he was not using his hocks properly.
YouTube - Tigger first lead changes
I found very little in this video that I would call smooth.
Horses moving with this level of impulsion rarely win WP at the national level. They have as you said shortened the stride but are they collected...not in the least.
Wwith 4 beat canters and jogs barely above a shuffle the lack of impulsion negates the term "collected"