We must take three things as truths.
You cannot influence a grounded leg.
The rider's leg will influence the opposite of the horse's body, just as the rein action will also.
There is no such thing as '' sponging''...
Why are these true?
When I back a horse, I look to see which leg is slightly out front, and give a light pull on that rein while settled in the saddle - and that leg, grounded tho it may be, moves back. Then the other side. It seems entirely natural to the horse.
Also, the ex-military guy in me says that there are two types of commands - preparatory command and the command of execution. (Drill Commands
) When training a green horse, I understand that there is no such thing as a preparatory command, because the horse doesn't understand the command of execution yet. However, a trained horse certainly can anticipate, with an off-time cue acting as a preparatory command.
Of course, the key to that is finding an ex-military horse...
Joking aside, at a higher level of riding, and if the horse is not allowed to anticipate, then I can see where that would be bad. For many of us, however, it is normal. I may try to time my leg cues to be in rhythm with the horse, but I can neck rein without worrying about which leg is where - that is for the horse to worry about. It is like head position - for many aspects of riding, I want
the horse to decide where to put his head.
As for my objections to the other two...that should be obvious...