Anti-rotation, anti-extension, single-leg balance. The part nobody trains until something tears.
For ten years I trained "core" as a synonym for "abs" — crunches, sit-ups, leg raises. Then a physio watched me serve and said the reason my lower back hurt at the end of every match was that nothing between my ribs and my hips was actually holding me together. Core is anti-rotation, anti-extension, anti-lateral-flexion. The visible muscle is the smallest part of the job.
After three rounds of trying complicated programs I cut the core block down to five moves that I can run in fifteen minutes with no equipment besides a mat and a band: dead bug, bird dog, Pallof press, side plank with reach, and Copenhagen plank. Each one trains a specific anti- pattern. I do all five, twice a week, every week. No periodisation. No deload. Just five moves.
Stand on one leg, close your eyes, set a timer. Most adults under 50 should be able to do thirty seconds. I could do six. For two years I thought my forehand was timing-related; turns out my forehand was balance-related, and my balance was non-existent because I sat at a desk all day and trained both legs symmetrically every gym session. The fix took six months and one minute a day.