The McGill Curl-up is an isometric trunk exercise with reinforcement of a neutral lumbar spine and pelvic floor tension.
Lie on your back and bend one knee to place that foot flat on the floor while keeping the other leg straight. Place your hands under your lower back to support your natural lumbar curve. Take in a deep breath, tuck your chin slightly, and exhale as you tighten your entire trunk, including the pelvic floor, and crunch your abs to bring the bottom of your ribs toward your pelvis—this will be a minimal movement but should separate your upper and most of your mid-back from the floor and push your lower back down into your hands. Hold this position for 10-15 seconds, or for a series of 3-5 controlled breaths. Switch legs and repeat.
The McGill curl-up crunch is a good combination of continuous ab tension like an isometric hold and minimal motion for the abs while maintaining neutral spine curvature, and as a way to practice and reinforce trunk bracing. It can be used for a low-demand ab exercise to reduce difficulty or to allow very high repetitions, and it can be used in training preparation to get the trunk activated without excessive fatigue.
The McGill curl-up should generally be done for 2-4 sets per leg of 3-5 breath cycles or 10-15 second holds. This is a good exercise for training preparation, or in addition to heavier, lower-rep or more difficult ab work in a given workout or in a circuit.