BILL– “Lazy” is a state of mind intrinsic to some horses, a quality which may or may not be alterable but within limits can be tolerated and worked with. “Behind the leg” is a state of training–often transient–measuring the horse’s belief in and responsiveness to the aids.
“Lazy” has a negative connotation, but in fact, for riders who are either timid or overly aggressive, it isn’t necessarily a bad thing. Some riders are happier having to keep making the engine rev rather than worrying—as with a hot horse—that the accelerator will get stuck, and the horse will speed off out of control.
Many more experienced riders prefer a horse with more built-in “motor.” They would rather modulate their horses’ energy rather than constantly have to wring it out of them.
On a whole separate continuum is how in front of or behind the leg each horse is. Two factors are involved: does he understand the concept and does he care? As the questioner implies, this whole issue is complicated by the fact that many novice riders don’t really understand the idea themselves and are unable to gauge whether their horse is fulfilling the requirements. “In front of the leg” can’t be measured by speed itself. A horse which is properly on the aids is in front of the leg at the halt and even at the rein-back. Meanwhile, another horse careening down to an open ditch at 450 meters per minute, only to catch sight of the obstacle six strides out and come skidding to a “reining horse stop” is anything but in front of the rider’s legs!
The only way for a rider to know is to test, test, and retest the horse’s response to the leg, both forward and laterally. He must answer it; he must also wait for it. That’s one reason we employ the multitude of exercises we use which shape, balance, and motivate the horses we’re on. Your newly adopted credo—rider to horse and horse to rider—should be Do ask, Do tell! Apply it liberally.
One other quality which ties this all together is generosity, not a technical term but an indicator of whether your horse’s personality is of the “Oh, gosh, Wilbur, I’ll give it a try” variety or the Bad Boy “betcha can’t make me” type. Don’t underestimate what a difference this can make in how all the rest plays out!
As for the final part of the question—it was posed, I know, rhetorically by a Glass Is Half Full professional. While she may be disconsolate at her students’ assorted failings, I’m a perennial optimist. I look at my riders’ fits and starts and unfulfilled expectations as my ticket to job security and long term employment!