I can't tell you how many times I've read threads on this board about hunters, jumpers or event horses where someone expresses the opinion that dressage is critical to their training, and I say to my computer monitor "NO, *flatwork* is critical; you don't want a hunter, jumper or event horse working in collection!"
Flat Work is just another name for Dressage. When we say "Jumping Is Dressage With Speed Bumps" we are not saying that you should be going around the Jumper Course or Hunter Course like one would while riding a 3rd level dressage test.
When I clinic with riders who are far above me in levels, I hear them stress how important basic dressage is for the horse and rider. I hear 3 star eventers, 4 star eventers, riders like George Morris and Jim Wofford and David O'Connor and many other big named riders saying the same thing - how important dressage is for the jumper.
Heck, there have been many articles in the Practicle Horseman about how important Dressage is for the Jumper and the Hunter.
No, you don't go around in a collected frame like one would in a Dressage Ring - that's rediculous. Your horse has to be in Jumper or Hunter frame, but they cannot be flat, they cannot be heavy, they cannot be moving around in different rhythms, they cannot come in unbalanced nor can they not come in unresponsive to the riders aids - seat, legs, upper body.
You don't go around in a dressage seat, but the aids, and riding seat into legs into hands is still just as important in the Hunter and Jumper ring, as it is in the Dressage Ring.
So yes, Jumping Is Dressage With Speed Bumps - because you have to have a horse that is rhythmical, you have to have a horse that can transition between going long and coming back down under you. You have to have a horse that is responsive to aids and that is round
*meaning back up into your seat* and your horse has to be on their hind end and not plowing around on their front ends.
You achieve that through dressage. Afterall - Flat Work is Dressage.
We are not talking about how a horse would go around in a 2nd or 3rd level dressage test - we are talking about the dressage fundamentals needed to accomplish that hunter round and jumper round well - via dressage.
If Grand Prix Jumpers spend the majority of their time emphasising their training on Dressage, there's a reason.
Of course, an Eventer such as myself will view Dressage differently than a Hunter, because we have to face Dressage in our daily training due to how important that score is for us in competition.
I invite you to read George Morris's column in the Practicle Horseman Magazine dated April 2009 titled "Dressage For Jumpers" where it discusses about riders of George Morris's clinic spending a full day doing only Dressage with Olympian Robert Dover where he works with them on a Balanced Seat, Forward And Strait, Balancing horse through rider from, and Basic Principles Of Riding - to take that emphasis of dressage to improve their riding in the Hunter/Jumper ring.
Yes, Jumping is dressage with speed bumps. When working on your flat work or dressage - you want balance, you want impulsion, you want lengthenging and shortening of strides, you want a horse light on their forehand and working off of their back, you want the horse to be respsonsive to your seat, legs, and all aids - and then to beable to transfer that into the Hunter/Jumper Show Ring.
Yes, Dressage makes your sport that much more tuned and refined.