I douse everyday (initially not for IS purposes, but for health purposes--yes..I got it from those crazy Russians while I was doing Systema lol) and I found that after every douse I had a greater awareness of the "skin" under the skin.
yup, do the same thing. most of the time for me is the psychological of "crap! it's cold. do i really want to do this? why am i doing this again?" sort of thing. i remembered when i live in Minnesota, folks would go into the sauna for awhile, then got out and jumped directly into a pile of snow in the winter. i wasn't that crazy so i never did. i went to an aikido seminar one time and we stayed at a hotel that has the jacuzzi next to the pool. so i alternated between the jacuzzi and the pool, and my body didn't ache as much during the whole seminar. methink, going from hot to cold sort of trigger the anti-inflamatory process inside the body similar to ice your joins or injury to keep the swelling down. don't know how much of the icing the body, misogi, stand in the cold fall water, wading in to cold river, and so on would contribute to internal training. it could very well be external training like the muay thai fighters who pounded their legs and body on regular basis to toughen their body. or misogi through heavy consumption of spirits.
i stopped doing this sort of thing for awhile now, because i found out that cold water and my manhood just didn't get along. i had problem finding it afterward, even with a GPS, flood light, and a magnifying glass. i felt less manly and self-conscious. bad enough that i wear a skirt in front of folks. this cold water thing just not cool!