He's thinking of back pressure from a clogged cat. When this happens,
exhaust gas isn't scavenged from the cylinders and power loss results. I
once drove a BMW with a clogged cat and the damned thing wouldn't climb
hills faster than a '50 Ford.
The easiest way to check for a clogged up cat is to use a simple vacuum
gauge connected to the O2 sensor port. Unscrew the sensor, and screw in an
adaptor. Connect the vacuum gauge and bring the engine up to 1500 rpm.
Normal reading is 0 psi on the gauge. If the pressure rises above 3 psi,
the cat is clogged.