It's pushing or driving your horse forward using leg pressure while holding or keeping them from picking up speed and moving too far forward by using bit pressure. The end result is that the horse moves their rear more under them and rounds at the poll and withers which rounds out the top line.
A good place to start is teaching the horse to give to pressure at the poll.
You can start on the ground by just pulling down/back on the lead rope/halter and releasing when the horse lowers its head. That is the beginning of collection. Once you have that done you can transfer it to the saddle. While sitting in the saddle pull back on the reins (while the horse is standing still) when the horse gives to that pressure release. Then start working on it in while in motion. Driving from the rear while at the same time getting a break at the poll to a vertical or near vertical position.
The head should be carried on the vertical with a bend at the poll when in a collected frame
Thats a very simple explanation for a fairly difficult thing to achieve :lol: