linch-pin
舝 refers to a linch-pin, a mechanical fastener used to secure a wheel to an axle. Its meaning is narrow and technical, centered on this specific hardware item.
The historical origin of 舝 is uncertain. It likely combines elements related to fastening or a wheel, but the exact development is unclear.
The top 巛 looks like flowing lines, the middle 冖 is a cover, and the bottom 㐄 and 厶 suggest a pin or wedge. Imagine a cover being secured by a linch-pin that holds everything together.
For カツ, picture a linch-pin being hammered in with a sharp 'katsu!' sound: katsu -> カツ, and the pin locks the wheel in place.