The diagonal is the leg you post to while trotting in English. You "rise and fall with the leg on the wall" - when the leg against the tail is forward, you should be up. If you are following that leg then you are on the correct diagonal. If you're not then you are on the wrong diagonal.
Now if there's no wall (like if you're on the trail) you pick a leg and go with it. If you're going for extended periods you want to swap. I usually hear diagonals referred to left diagonal and right diagonal in cases like that.
The counting method only works if you know which leg you're starting the count on. If you start counting when the right leg is forward the those numbers are backwards.
To swap diagonals, you stay down for 2 beats then resume the normal up-down.
You have to develop the feel for it. You will learn how to feel when your horses shoulders are moving and this when to post. Posted via Mobile Device