The geometry pushes the driver back out
A Phillips driver does more than meet a cross-shaped slot: its angled surfaces turn twisting force into a small upward push. As resistance rises, the bit can climb out of the recess—a behavior called cam-out. Once it slips, the driver’s edges scrape the slot. Each slip makes the fit looser, so the next attempt is even more likely to slip and damage the screw further.
A loose fit makes that cycle start sooner. The wrong driver size may touch only a few edges; a worn bit or a screw already chewed up offers even less grip. Pressing the driver firmly into the screw helps, but cannot restore metal that has already been torn away.
A production tradeoff, paid at repair time
Cam-out can be useful on an assembly line. When a powered driver meets strong resistance, slipping can limit how much extra torque reaches the screw, reducing the chance of driving it too far or damaging the part. The same feature is frustrating when someone later needs to remove that screw: the tool may slip before it can loosen it.
For a stubborn Phillips screw, the practical goal is to keep the bit fully seated: use the correct size, a bit with crisp edges, and steady pressure while turning. If the head is badly damaged, repeated attempts with a poor fit usually make removal harder. The small cross that speeds up fastening can therefore determine whether a repair stays simple—or turns into a job of extracting a screw with no usable slot.
