This trick can be performed in both directions (Whip to Straddle or vice versa). It's most often done at the back of the swing, although is also possible at the front. When learning, it's best to try at the frontend after a turnaround, as doing it above the platform is dangerous if you mess up and fall.
Whip to Straddle Whip
From a Whip position, pull towards the bar, let go, open your legs and grab the bar between your legs then hold the Straddle Whip position.
Straddle Whip to Whip
From a Straddle Whip position, pull towards the bar, let go, close your legs and grab the bar on the outside of your legs in a Whip position.
You can find out more about this trick in my books:
Thanks to Thugz Bunny and Anna for providing videos for this page.