When I first started jerking off as a young kid I loved doing it prone - I would slide my cock against the pillow. My pillow became so stained and crusty with cum. When I grew older I discovered the joys of fucking my palm, but now and then, when I do, the prone position still makes me cum buckets. I think its because my nipples are simultaneously stimulated as my glans that way, and my orgasms are therefore stronger.