Pavel said:
Steve, I bet folks serious about flexibility would love to hear you expand on the foot position.
It's hard to see his feet when the split is at its maximum, but the thing that impressed me the most was that his feet weren't point completely out to the sides. It looks like he still has some of the bottom of his foot on the platforms on the trucks' mirrors. The fact that there are platforms there is one example of smart planning, but the fact that he is able to keep pressure on the bottom of his feet make it into a different kind of split.
It goes without saying that I have not attempted anything like this, which means I have no standing at all to say anything about this amazing feat of strength and flexibility, but I have done enough splits to know being able to push out on the bottoms of your feet against resistance (in this case the platforms) makes it different than being in contact only with your calves and not your feet, and my suspicion is that this is, in fact, easier overall.
If I may be so bold as to offer some general observations about side splits, suspended and otherwise:
One of the secrets to performing a suspended split is, in fact, manufacturing for yourself that push through the bottoms of your feet even when there is nothing to push against.
Often overlooked is the strength aspect of splits. It's not only about learning to relax in the position, it's also about being able to produce force - to, if you will, demonstrate to yourself your own strength so that your mind will allow your muscles to relax in that position. The knowledge that you are able to use your muscles, even if only to flex them hard, in any position can be a key component of progress in extending one's range in any plane of movement.
The push through the bottoms of your feet will, at the very least, help you separate the pelvis, but it will also help you keep the necessary tension throughout the legs and hips when your butt is in mid-air. Even if you're working on a side split sitting on the floor, it will help - push hard against the bottoms of your feet, and cycle the hard pushes with exhalation and relaxing without giving up your position. Practice using your legs and hips like you would at the top of a deadlift: zip up your thighs and knee caps, flex your ankles hard - since your feet aren't pushing against the ground you need to manufacture that yourself - and use your glutes to drive your pelvis forward to increase the straight-line-ness of your leg position.
(For anyone reading along, I have a web page with all of Pavel's flexibility resources plus a few others including Jon Engum's "Flexible Steel," here: http://www.kbnj.com/flexguide.htm)
Pavel, I hope you will add/amplify/correct as needed.
-S-