several months ago it was sensei explaining something & uke gets bent over backwards & goes up on toes. sensei explains "lifting the hips" in that position "to get the head closer to the ground" so it doesn't have as far to fall.
so i started experimenting with up on my toes in backbend & try to do that raise the hips thing. hard. calf muscles get sore after few days, transient cramps in the muscles of the sole of the foot.
i found thoughts of running on toes. i have been a heelstrike runner. vibrams really demonstrated the problematics of that style. i considered the aiki approach would be a lower angle but was still imagining only a heelstrike. bringing it up in class with a more experienced person who came back with tales of a guy who runs on his toes alone. so i tried it.
i had ortho leg problems when i was a kid. special shoes: Thomas heels. I was told to run "on my toes" but i didn't want to run anyway & running that way felt weird.
at the beach i started watching runners' feet, found several running on their toes. so i learned how to do it. on the beach i could go back & look at my footprints in the sand to see what i'd been doing. mostly flat, occasionally toes only. sure is a different feeling from heelstrike.
the main contact point of the feet changes from the heel to the ball. the toes want to spread out to "grab" the floor. the heels rest lightly, the weight spreads out through the rest of the foot. because of that spread out weight there seems to be enhanced stability: four zones of weight bearing: inner toe, outer toes, ball, heel, instead of just heel. because of enhanced mobility of the foot there is increased freedom of action of the ankle, that's always good, having choices.
of course the rest of the body has to be properly draped over the feet, 0-0-0.
speaking of which there was an incident where i got bashed in the nose in class. it was nage's "fault": clumsy approach, contact with wrong part (nose), heedless follow through with unnecessary force & bad vector, possible psychological aspect. my job as uke is to make lemonade. after several days of thinking "how can i approach the issue with the person" mixing up with "what can i do to prevent this in future" this came up: keep the head back, go up on toes, lift hips. i could have jumped up, as sensei once mentioned, specifically to keep uke's nose from being broken "again."