Dunno about the mechanics of the leg-kick, but if Buddy’s belly doesn’t trigger it, try scratching him on the back, right above his rear legs… that oughtta get you a standing-up version of the leg-kick. If you find the back trigger and the belly trigger, scratch them both at once, sometime.
Do they all universally love tummy rubs? Nearly all the times I’ve started petting a dog who’s into it, they roll over on their backs which I assume means they want tummy access.
IME they do this when you are scratching/tickling a spot that they have recently been scratching with the leg in question. They have some sort of “muscle memory” from their leg action, and scratching the right spot triggers this.
I thought it was sort of a reflex action, like when your doctor tells you to cross your legs and then taps right under your knee cap with a hammer or his hand and you’re leg goes up.
Several of my dogs did that, and now my cat (rarely) does, too. Rather than muscle memory, I always thought it was because the animal really liked what you were doing and involuntarily wanted to “help,” but then would check the motion of the leg because he didn’t want to interrupt you.
Gotcha all beat. I once knew a HORSE that did it when you curried his belly. Just like a dog he would stretch his neck out and half close his eyes and his hind hoof would start kisking.
It was adorably cute but you had to be careful where you stood…
My mom’s old shih tzu would, once you got her going like that, keep her leg pumping and scratching said spot herself (a la a perpetual motion machine).