Another useful mental image to encourage the correct relaxed whipping feeling is to imagine that your hand has been dipped in something sticky, like honey, and that you are trying to flick it off. Making this flicking movement with open hand (ending in sword hand at completion of the stroke) instead of fist also helps one to learn to be relaxed and quick. Then this feeling can be transferred to the block when made with fist. The act of clenching the fist can retard one's movement in many techniques, not just gedan barai.
In Karate-do Kyohan, Funakoshi Sensei emphasizes use of both hands and that from their position crossed in front of body, the feeling is as if ripping something apart with the opposing motion of blocking hand and hikite.
