Disc in the middle of the field.
Open side handler defender poaches into the open side lane. Break side handler defender stays tight.
Having the open side handler defender in the lane makes the upfield open side throw much more difficult. This defender should be "getting big" by holding his arms out, and watching for the handler to make a throw in his direction.
If the offensive team swings to the open side handler, the defender who was poached has to hurry over to set the mark.
The defender on the middle handler now needs to be very careful not to get beat on a quick throw and go up line cut.
Disc back to the middle of the field...
If the offense gets the disc to the break side handler, the defender on the middle handler now flexes into the open side throwing lane. The defender on the open side handler (3) pinches in on the middle handler to help defend a quick swing.
If the offense swings back into the middle of the field, the defender on the middle handler gets back on the mark, and the defender on the open side handler flexes into the open lane as before.