There is an old saying and I am sure most have heard it "Rise & fall with the leg to the wall". When I first starting riding english a few decades ago, and the instructor said this, it was like "huh, which leg to the wall, and I gotta fall somewhere?" I learned to cheat, look down at the horse's outside shoulder, when it goes forward, you rise up. Even after all these years, I have to slightly cheat for the right diagonal, left one I can tell.
Would love to see a video where the rider doesn't out rise the trot.. but anyway!
When you rise the outside leg goes forward, when you sit the inside leg goes forward, When you change direction you sit two and start rising again :)
Which leg you rise up to effects your weight, if a young horse is drifting to the outside if you change your rising (so when you rise up the inside leg goes forward) it actually changes your weight to the inside helping the horse balance, once balanced and not drifting you simply change back!
Very useful tip, works absolute wonders! This is where riding with your seat and legs comes into effect! :)