The triangle progression marking scheme is a way for defense to effectively catch up to their man and eliminate the quick throws that some teams use to move the disc up the field in a short amount of time.
The goal of the scheme is to force the throwers to pivot or move around more than they should. Keep them moving more and more which makes them waste more energy and prevents them from thinking about the best throw to make.
The first step is to prevent the quick or the break side throw. When 1 catches up to H1, they should move in and prevent the break throw - Assume forcing flick.
Once the break throw is blocked up, 1 should move back a bit to prevent the huck throw (long bomb, hail mary, deep throw).
After the huck is eliminated, 1 should take a step forward and continue forcing flick. Notice, the triangle movement that 1 makes while marking H1.
1 Should continue marking in this type of progression not to block the throw, but to make H1 move more than they need to and make their throw sloppy.
The triangle progression can be done in the other direction. Instead of 1 moving in a clockwise direction, they can also move into a counter-clockwise direction. They block the open throw first, then take one step to their right to prevent a break throw, and then move a step back to prevent the huck throw . All by doing this in a triangular pattern.