Active stabilisation of abdomen, back and pelvis — the bridge that lets force travel between legs and arms at all.
Without core tension, force leaks. You can push as hard as you like from the legs — if the middle gives way, only a fraction arrives at the top, in the gym just as much as on a paddle, a kite bar or a boulder wall. The usual mistake is confusing tension with holding your breath or sucking the belly in. What you want is 360-degree tension: breathe in slightly, brace all the way around as if bracing for a punch, ribs down, pelvis neutral — and still be able to breathe. If you can't speak, you're straining rather than stabilising.