Roll the ball in and pike your hips to the ceiling.
Lift from your abs into the pike, keep your legs straight as the ball rolls in.
Rolling the ball from your shins toward your hands while your hips rise to the ceiling makes the abs lift a huge portion of your bodyweight through a long arc, and they do it while stabilising a ball that punishes any wobble. It is essentially a moving plank that finishes in a near-handstand pike, which is why it sits at the hard end of ball core work. Earn it after roll outs and knee tucks stop being a challenge.
If a solid stability ball plank or knee tuck is still hard, yes, build those first. The pike demands shoulder strength and core control at the same time, so progress to it rather than starting with it.
No, keeping the legs straight is what makes it a pike. If they keep bending, you are doing a knee tuck, which is fine, just treat it as the stepping stone and keep working toward straight legs.
Shins for the standard version. Starting with the ball nearer your feet lengthens the lever and makes it harder, nearer your knees makes it easier, so slide the start point to match your level.
