The flat chop is only the first stage of the process. The apical growth is super strong, so the flat chop is designed to inhibit the callusing near the new leader - it more or less provides a "shelf" the callus has to roll up over which is not as easy as rolling past an angled cut (causing reverse taper). After year one of regrowing the new leader, an angled cut is made from halfway across the flat chop away from the leader, as you would any other angled cut. Since you're regrowing a strong leader using the grow and clip technique, you want to keep the callus from rolling too fast and the shelf of wood it has to get past does the trick. By year three, you can go in and carve out the excess wood off your shelf and allow the natural healing process to continue. By that time, you generally have a fine tapering transition into your new apex.
Zach