Legs up the whole time, abs never get a break.
Keep that bottom leg from dropping. Constant tension is the whole point.
Keeping both feet off the floor for the entire set means your abs are holding an isometric before the first twist even happens, and every pedalling rep is layered on top of that constant tension. The raised-leg position also stops you resting the non-working leg between sides, which is the sneaky break most people take in regular bicycles. It earns its spot as the harder sibling for when standard bicycle crunches stop biting.
In the standard version the extending leg can hover low or touch down, giving your abs micro-breaks. Here both legs stay up with shins parallel to the ceiling, so the tension never switches off from the first rep to the last.
Holding both legs up pulls on the pelvis, and if your abs cannot counter it the low back lifts. Press your lower back gently into the floor, keep the knees more bent, and shorten the set before your form leaks.
Work in time or reps, but end the set the moment the bottom leg starts dropping or the low back peels off the floor. Twenty crisp rotations beat fifty sloppy ones.
