Unstable base, loaded crunch, real abs.
Use the dome for a deeper stretch at the bottom, don't rush the range.
Draping your lower back over the dome lets your abs work through a longer arc than any floor crunch, starting from a genuine stretch before contracting to lift your torso. Holding the weight overhead with extended arms pushes the load far from your hips, so every degree of the crunch costs more. It earns its place as one of the few crunch variations you can load progressively while still training the full range.
Leverage. The further the load sits from your hips, the harder your abs work for the same weight, so an overhead plate or dumbbell turns a light weight into a serious crunch. Arms stay extended, the weight tracks over your head as you rise.
Range. The floor stops your spine at neutral, but the dome lets you extend back over it, so your abs get stretched before they contract. That stretch position is where this exercise beats floor work.
Light enough that the weight stays over your head and your reps stay smooth. If your arms bend or you jerk off the dome, drop the load, the leverage does the work here.
