So with the hip , there is a little "nub" as I like to call it at the end of your femur. (see the picture, not the "ball" but that boney protrution)
http://health.rush.edu/HealthInformation/graphics/images/en/7015.jpg
When you do the straddle split, that "nub" has to move around the pelvis. The way that you this is rotating your hips forward, like arching your back, otherwise in many people (like myself), it "catches" on the pelvis and doesn't allow the femur to extend any further. That particuluar stretch pictured is my prefered method of straddle training because my femur rotates inward (my kneecaps touch when I'm standing with my feet together), so I have to hyper arch to allow that bone to move. So by placing your hands really close to your body, pulling your shoulders back and down I am able to better get that bone around.
Also, many people hold the stretch for long periods of time in this stretch. I don't feel it's the best way to do this particular stretch. In a straddle stretch your brain gets a "danger" signal, like it thinks it's fallen into this position, it then sends a signal to your nerves/muscles to tense everything up to protect itself and tells you there is pain. As long as that happens your won't be able to fully relax into the stretch.
A better way is to get up to the wall as close as you can without pushing it (just getting into position), then push for 2 full seconds (don't forget to hyper arch pulling those shoulders back and down!), then pull completely out of it for at least 2 full seconds (I come out for about 4-5 sedonds because I have fibromyalgia and nerves freak the eff out more than most lol), then push again for 2 full seconds and out again, 2 full seconds. 2 seconds isn't enough time for your muscles to fully freak out, allowing them to stretch further with each two seconds! I am able to get about a 6 inch gain for that stretching period from the first stretch to the last. Now don't excpect to go right into the furthest stretch point the next time you try, straddle is a tricky one for many people and can takes a looong time to get it. Patience is key!
HTH!