Those are called myoclonic jerks. I've had those all my life, and they seem to have become more frequent over the past few months when my twitching started -- I sometimes get whole body jerks. They are a very commonly reported occurence on this board and are nothing to worry about.
ETA: I don't get large twitches like then when I'm fully awake -- yet *g* -- but many other people here report large, visible twitches and jerks when they're awake.
ETA: I don't get large twitches like then when I'm fully awake -- yet *g* -- but many other people here report large, visible twitches and jerks when they're awake.