Day 20 hands the core a hybrid. Three 20-second stations: sit-up punches and sitting punches - Day 4's opening pair, now a third longer - and a new finisher that alternates scissors with sit-up punches inside a single interval, at 3, 4 or 5 rounds.
The hybrid is the interesting one. Scissors work the lower abs and hip flexors with the legs long and crossing; sit-up punches work trunk flexion and rotation up top. Alternating between them within one 20-second window means the entire abdominal wall trades the workload back and forth with no seam in between - the legs rest while the trunk punches, the trunk rests while the legs scissor, and the abs as a whole never do.
Keep the scissors low and straight-legged, crossing at the ankles just above the floor, with the lower back pressed down. When you transition to the sit-up punches, do it without parking - flow from legs to trunk and back in one continuous piece of work. The familiar stations keep their standards: full rise and full punches on the sit-ups, long spine and honest lean on the sitting punches.
Scissors below, punches above, and not a single second where your abs are off duty.
Make it Harder: Keep the heels hovering off the floor for the entire final interval - through the punches too.
Make it Easier: Bend the knees slightly during the scissors and shorten the leg lever.
