Set-up: Place or arrange a stable elevated surface - for instance a plyometric box or anchored bench - slightly closer than body-length away from a wall. Place your forearms on the surface, as in a traditional plank. Keeping your back neutral and bracing hard through your core, lift your feet up one at a time and press them against the wall, forming a plank through your body.
Lift: Now elevated and with a very stable brace through your core, remove one leg from the wall and raise it towards your abdomen. Hold this position for the prescribed time, then repeat with the other leg. Do not allow your butt to pike up or your lower back to round up or sag.
Repetitions: Once both legs have been raised and held, this counts as one repetition.