Take a neutral grip on the parallel bars, hands underneath your shoulders, arms straight, & triceps engaged.
Depress & protract your scapula, round your back (drive your shoulder down away from your ears, and spread your shoulder blades)
Position your feet slightly in front of you, tuck your knees tightly, & raise them as close to your chest as possible.
From here extend one leg, while ensuring that the other knee stays as high as possible for the duration of the hold. Squeeze the quad of your extended leg, and try to keep that leg extended as high as you can, hold for time and then switch legs.