TRX Suspended Jackknife
Attach a TRX to a pull-up bar and let the stirrups hang about three inches off the floor. Place your feet into the stirrups so that the top of each foot is facing downward. Position your body in a push-up position with your hands slightly wider than shoulder-width apart.
Brace your core and drive both of your knees towards your chest. Pause when your knees are directly under chest and reverse the direction back to starting position.
Don't let your hips sag. Maintain a straight line from your head to heels.
Keep your core braced throughout movement.
Squeeze your abs and glutes to maintain stability.