Adding a defender onto B forces more realistic throws. Any throws that hang or are misread will likely lead to a contest between defender and receiver; this is what you want to avoid!
This is ideal for practicing throwing continuation after breaking the mark
A handler, A, starts with the disc on the sideline, marked by D1. A cutter, B, starts 5-15m downfield and 5-10m infield from B.
D1 > A > B