A full ab contraction loaded up for serious strength.
Curl up segment by segment, don't jerk yourself up with momentum.
Holding a weight on your chest turns the sit up from an endurance drill into strength work, loading the abs through their full job of curling the torso from the floor to upright. Bracing and the chin tuck keep the spine flexing segment by segment rather than jerking up in one piece. It earns its place because abs grow like any other muscle, with progressive load, and endless unweighted reps stop delivering long before a hard loaded set of ten does.
Hugged to your chest is the standard and steadiest option. Holding it further from your body, like behind your head, makes the same weight much harder, so treat position as part of the progression.
Anchoring lets you go heavier but invites the hip flexors to take over. Unanchored keeps the abs honest at lighter loads. Either is fine as long as you curl up segment by segment rather than yanking.
Sit ups take the trunk through a longer range, all the way to upright, while crunches isolate the top portion. If you want to load the abs progressively, the weighted sit up is the stronger tool.
