The first is a belly lift.
Placing one knuckled hand - or stiff fingers - on either side of the tail, at the point of the croup, about 1" to either side of where the tail begins. Draw a straight line down, with moderate to heavy pressure, to just under the point of the buttock, or about halfway down the gaskin. Watch your horse's withers and back while you're doing this; every horse will have a slightly different trigger point. As you trace down, his back will lift. When you reach the gaskin, it will be about as high as it can get.
I started doing 5 of these, and now I do 15 every time. I hold the lift in the back for a good solid 2-3 seconds. You can also adjust to focus on one side or the other depending on how your horse is standing, or where he's turning his head. A head turned to the left will give extra lift to the left side of the withers; the opposite to the right. Ideally, they should be square for most of them but it's fine to turn their head for some of the exercises if you're trying to even out an imbalance.
This isn't just a back exercise, either; though you can't see it from the back, the back lift is at least partly because this technique causes the horse to tighten his abdominal muscles. It simulates crunches in humans. So it does double-duty, lifting the back and tightening the stomach.
The second exercise is a sternum lift.
Reaching underneath your horse's chest, find the sternum with your fingers. It'll be about midway, and when you push up through muscle/fat (and in my case, winter fuzz) you should feel a clear thin line of bone. Using stiff fingers, dig into that bone, perhaps wiggling your fingers a bit, and keep your eye on your horse's back: it will not rise as obviously as with the belly lift, but it will gradually fill in and have more of a "finished" look than with the first exercise.
I do these for 10 seconds on, 10 seconds off, working up from 3 the first time to 5 now. This one targets different muscles (though there is some overlap) and activates them in a different way. In a way, this one teaches them to hold the lift themselves: watch closely, and you'll see how long they hold after you remove your hand.
We'll have to wait for updated topline photos in another few weeks to see if these are helping along with the rest of the work we're doing, but judging by the evidence of my eyes, and the way the muscles are being used in these exercises, I'm very pleased with them.