While playing on the parallel bar (chin up bar?) the other day, J was swinging his legs. He let go before I was able to get to him, and SPLAT, he had a face full of sand and the wind knocked out of him (I'm still a big sand as a landing material proponent). He's ok and wanted to continue playing after a few snuggles.
We've been working on the only drop when your legs are perpendicular (straight up and down) from the ground and you aren't swinging. However, it's been a few weeks since we've been able to go to the park and he must have forgotten.
Here's what happens:
What J did the other day:
His center of mass still wanted to be directed downward (thanks to gravity). However, with his feet still swinging backwards, he ended up with a moment (rotation) which resulted in him splat-faced on the sand. I don't recommend experimenting with this way, as it really hurts to get the wind knocked out of you, and it doesn't taste good to eat sand.
However, you might try falling a different way (in a safe environment - meaning on soft/safe landing materials - sand, gym mats, etc. - and not from very high). In my family, we come with built in cushions, so falling on our behinds is no big deal.
J demonstrates the slight forward leg swing (think opposite of the second drawing):
When J lets go in this position, he's going straight to his bottom. Please note, I would recommend a more centered position on the bar, as scraping the playground on the way down is also no fun (he didn't drop from this position). Also, if you swing too much forward, you'll end up on your back (also, probably with the wind knocked out of you) and/or hitting your head.
I might not recommend going out and seeking your own answers to this experiment, but I'm sure if you have a monkey like I do, he/she is experimenting on their own and figuring it out (physics). Now you might be able to explain why a straight drop is recommended and not to let go during a swinging motion.
P.S. My biggest fear is that J returns to the ledge after hanging and lets go with his center of mass still not all the way on the ledge, falling head first off of the playground equipment. So, yes, I encourage straight drops. Practice makes perfect (or at least a little more relaxed mommy).