Charge Pump: A small, simple internal pump (typically a gerotor pump) that does two things: 1) It keeps the main closed loop full of oil to prevent air bubbles (cavitation), and 2) It circulates oil through the filter and cooling fins. It's essential for the transaxle's longevity.

Bypass Valve: This is the lever or knob on the back of the mower. When activated, it opens a passage that lets oil bypass the motor. This releases the hydraulic lock, putting the transaxle in a true neutral so you can push the mower by hand.