This is my saddle, plain. It is 16", which works OK for me with a padded seat but too big for me as a slick seat, slick swell saddle:
The problem is that it is like skating on ice. At any sudden change in the horse's speed or direction, either I'm braced against that motion with the stirrups...or my butt slides. I can recover, but any time my rump is sliding, I'm not moving with the horse. In cantering, some folks say to scoop the saddle, or to "polish it" with your seat. But the truth is my saddle moves with the horse, and any time my rump is sliding, I'm NOT with my horse.
What shocked me when I tried to shrink my saddle with a sheepskin was how incredibly easy it made it to simply move with the horse. No sliding. Even if Bandit leaps into a walk to canter transition - which he does when we are in the arena because the arena is small and he has a tough time with the tight turns - even when he leaps into it, my rump just follows the motion. I can be relaxed, doing nothing with the stirrups, nothing to anticipate the change - and my body just goes along with the horse. That is probably the closest thing I can do to imitate a spook-based jump forward. With the sheepskin, WE just leap forward.
I had people tell me it was easier to learn to canter bareback than in a saddle. If so, then it is probably because the horse's back is muscle and hair and it grips you the way the sheepskin does.
I don't know why. Not for certain. But I do know it makes staying very steady on the saddle and horse very easy. It has improved my cantering even when I take it off, because it taught me what the horse's actual motion was.
And where I first noticed it was when Bandit would slam on the brakes - something he has largely stopped doing. But when he did, I didn't slide forward. At all. It is like having a super-rough out saddle, covered in sticky stuff - except comfortable! The extended version gives grip to my thighs (not my knees). And it is automatic.
So I've taken to calling it butt velcro. I insisted my wife use it. I told my daughter after her first fall a while back that if she didn't want to use a helmet, then she WOULD use the sheepskin.
Once in a while, I will ride without it. That helps me to check my position and balance. But 90% of the time, I go with sheepskin - and plan to for the rest of my life. I consider it a safety device, and I think it helps more than a helmet does.
Of course, it doesn't rain here either...
...but it takes me about 2-3 minutes to put it on, and about 1 minute to remove...