For many years we've wondered, but here's the truth:

At Keirin School in Japan riders are trained to not release the handlebars when crashing.  This is to (in theory) protect their arms and collarbones.  Many keirin riders in Japan wear body armour that includes shoulder padding which protects them when they fall and they're trained to land on the padding not to extend their arms.  This is also why keirin gloves have armoured knuckes.  If you hold the bars when you crash, guess what hits the deck .. yep, your knuckles!

This is according to Josiah Ng who just got back from a racing tour of Japan where he rode some 90-odd keirins and had no crashes!