If going over the handlebars, putting out your arm, locking your elbow, then using your hand to cushion your fall puts you at a risk of a broken clavicle.
I once had a large stick go into my front spokes on my mountain bike and send me over my handlebars. I kept my hands on the bars and tucked in my head. The result was a half somersault from which I jumped up and did a tah-da pose. If he would've tucked in his head, you might have ended up clapping instead of laughing. With a little BMX bike, he might have been able to complete the loop and land back on his wheels. That would've been cool...