From sitting on the catch trap. Start your lock with a crossed grip, turn on the bar as you bring your legs down at the frontend. Swing on the catch trap as you would on the fly bar. Break well before the bottom of the swing and let go before the peak for a travelled layout. You need to let go before the peak in order to have enough travel to cross the distance between the fly and the catch trap.
You can find out more about this trick in my books:
Thanks to Ryen for providing this video