Weight overhead, full sit-up, full payoff.
Keep the weight locked overhead, don't let your arms swing forward to cheat.
Holding a dumbbell at arm's length overhead turns a basic sit-up into a long-lever core exercise, because your abs have to move your torso plus a load positioned as far from your hips as it can get. Keeping the weight stacked over your head also forces the trunk to stay rigid rather than jerking through the rep. It is a simple way to keep progressing sit-ups when bodyweight reps stop being a challenge.
Stacked over your head with arms extended the whole way up and down. If your arms swing forward toward your knees, you are buying momentum instead of working your abs.
Lighter than your ego wants. The overhead lever multiplies the difficulty, so a modest dumbbell held strictly beats a heavy one that turns the rep into a swing.
It is harder rep for rep, because the overhead position lengthens the lever instead of just adding load. Build up with the chest-held version, then move the weight overhead.
