Cady Goldfield said
Thank you for that, Jun (who babysits the play behavior on AikiWeb with the patience of a daycare worker!). I believe that over time, aikidoka, judoka and others come to realize that adding force to a breakfall is counterproductive. Odd how common sense ain't so common (tip of the hat to Mark Twain).
Cady, I'm not sure you undertand the physics of active and reactive forces. Force = mass times acceleration. Slapping the mat hard and fast is a countering force and lessens the fall. I was trained to break fall in a judo class where ground work was minimally taught. Most of the falls we took were high, hard falls.
I respect the opinion of Ushiro Sensei, but would add that the falls I took in judo were guided all the way to the ground - this means maximum force delivered by my body to the ground. I was taught that slapping was to protect the internal organs and spine, not keep my hand from stinging. Crossing the feet as Sensei suggests might work where there was less force, but in a highly forceful break fall - landing on the edge of the hip bone And the ankle is preferable to having the sacrum blown out of joint, or risk damage to kidneys.