(love your avatar image)
Ok, Turn on the forehand vs leg yield . (I won't address sidepass as I don't ride much Western and have never been formally taught sidepass.)
As for LY, it as much, or more, a forward movement as a lateral movement. It is just as important for the horse to take forward steps as it is for him to step over to the side. Let's use LY to the LEFT for this discussion, ok?
Here I am legyielding to the left; I take up a tiny bit more contact on the "inside", (inside of the minute bend), Rt rein and suggest with a "tickle" of my fingers that the horse give and make a very slight bend, really just tucking his jaw to the right . I put my right leg on at the girth and press gently. The outside leg goes back an inch or two and it is there to support the horse and remind it to continue going forward.
The outside rein stays on supportive contact. If the horse is minutely bent to the right, the outside,left rein with be on his neck and thus act as a bit of a "wall" to keep him from going too much sideways. The outside leg also does this. It stays constant on the horse's side draperd there, while the "driving" right leg will be pulsing with each step.
So, I put my active, inside leg on just as the horse's inside leg steps under. then, when he is stepping the left hind I kind of "hold" him with my left leg (not alowing him to swing to the left but to go forward instead) and "hold" him with my outside rein and think "Forward please". So, I ask basically for one step sideways, one step forward, one step sideways, one step forward . . . My teacher said if your horse loses it's forward and starts to angle outward to much, ditch the LY and reestablish forward thinking and try again. It's kind of asking for a sideways movement and then recieving that energy into the outside hand and channeling it into a forward step. 'over, recieve, over recieve" Sounds vague, I know.
The step over sideways in LY is not very big. The classical teachers of dressage would see little value in LY and would have students spend their time on Shoulder in as the more important trainging aid.
As for Sidepass, someone else will chime in.