In order for your lateral work such as shoulder-in, haunches-in, and half passes to be effective as collecting exercises, your horse must BEND.
Think of the following equation. Bend+Sideways=Engagement. (Engagement means the bending of joints. As your horse bends his hind legs, his croup lowers, and his forehand goes up--kinda like a seesaw or an airplane taking off.)
So here are some quick tips to help you determine if you're bending your horse correctly to get maximum benefit from your lateral work.
1. Shoulder-in--Look at the hindquarters. Are they parallel to the wall as if you're just riding straight down the track? If so, your horse is bending. If not, he's doing a leg-yield in shoulder-in position.
2. Haunches-in--Look at the forehand. It should look exactly the same as it does when you just go straight down the long side. If your horse's neck is more bent to the inside than that, you're fudging the bend. If his shoulders or front feet point out toward the wall, he's just doing a leg yield in haunches-in position.
3. Half-pass--Half pass is just haunches-in on a diagonal line. So the same rules that apply in #2 above, apply for half passes. To check yourself, go straight across the diagonal as if you're just changing direction. Then ride haunches-in for 3 or 4 strides. Then go straight again. The critical point is when you straighten your horse. Ask yourself if your horse has to swivel his forehand around to get it back on the diagonal line. If he does, he isn't truly bending in the half pass.
A Happy Horse