Bump is also called “Ward off” or “péng” 掤.
Maintaining the centripetal engagement while issuing power requires keeping the arm still (relative to the body) when the hips are moving.
Bump is done by rotating the pelvis around either the right axial line or the left axial line (which we sometimes conflate with the “axillary line”), and by keeping the shoulders down and keeping the core properly aligned.
The right axial line, in this case, is the line which passes through the right hip and the right shoulder. The left axial line passes through the left hip and the left shoulder. But pivoting around these, the body swings like a door. Whether the body acts as a class one lever or a class two lever, will depend on where the point of engagement is.
With slight modifications, this movement can act as a screw or a pulley. With further adjustments and proper core training, this movement (like many movements) can act like a Galilean cannon.