From the article I cited, since no-one seems to actually read it (
Shooting Illustrated | Failure Drill):
The first two shots are the same as the standard center of visible mass drill. The third shot is where the drills differ. The standard center of visible mass drill teaches you to continue to shoot at center of visible mass, while the failure drill teaches you to slow down, asses the situation, and shoot more accurately and effectively. The failure drill is designed to deal with an attacker wearing protective body armor or such heavy clothing that the bullets do not penetrate sufficiently. It is also designed to deal with an attacker that is high on drugs such that his body does not react quickly to the hits.
None of this has to do with moving or cover. You shoot while moving with both drills. You shoot at a moving target with both drills. You do not trade off accuracy for speed with both drills. If the attacker is behind cover, the head would be the most likely center of visible mass. If the head is not visible (which is a really weird use of deadly force situation by the way) then you shoot at center of visible mass.