What the? I've never heard of any Trainer discuss stretching your horses legs before riding. I can see pulling their legs up and forward to adjust the girth to prevent skalls - but not for getting a horse to round their legs over a fence.
TUCKING properly at a fence, comes by proper work through Grids. GM and other top level riders talk about raising the fences, too many school grids over too small of fences, which creates a "Lazy" jumper.
BUT!!!! Riders form on approach to the fence, the riders legs and seat makes a HUGE differnce as well. We must always remain supportive through our legs, aiding in lifting our horses backs and ribs up into us. We must keep them rounded and engaged through our seat and into our legs - engaged, rhythmic, on approach to the fence. Our horses must always be off their forehand.
When we see riders with incorrect leather lengths and unconditioned legs *Not wrapped around our horses girths* we allow our horses to go flat, on the forehand - which aids in a sloppy jumper.
That is what the 1/2 hour of Dressage Work is meant to do - To aid our postions, our aids. To allow our horses to stretch, to achieve balance, rhythm. Riders achieve control, tempo, fluid movement.
All this aids - in a better jumper.
At the show barn I used to ride at almost everybody did it.
Think about it athletes stretch themselves why shouldn't horses.
If a horse is stiff (despite a good warmup) they will be uncomfortable with pulling their knees up. If they already have had them stretched then they will move more loosely.
I was recommended to do this by a message therepest with my tb mare because of the tendency for tbs to pull their hamstrings.
When a hors pulls a tendon it because of the drastic flexon exhibited. If you get the horse used to the flexon gradually they will be able to do it without injury.
It is merely giving them the best chance of pulling their knees up. It is very effactive.