Ahh.. the topic of "dharmabation"!
I also masturbate frequently. However, my experience during the two ~10-day retreats I made, is that masturbation becomes quite irrelevant. The super-powerful concentration I get during such a retreat results in a state of mind when mindfully noticing the lines imprinted in my hand will be far more fascinating than any wank. Of a level of fascination, indeed, that I had only experienced on LSD before.
Retreats can be quite engrossing.
Alan:
(1) Is it possible to jerk off mindfully?
(2) Perhaps noting the Three Characteristics the whole time?
(3) Would this totally destroy the concentration and insight one had so carefully built up all day via heroic meditation sessions?
(4) Or is getting it done no real impediment to progress on the path?
(5) In the spirit of pragmatic dharma, I need to know the truth behind jerkin' and enlightenment...
(1) yes, and that's true for just about anything.
(2) Ha ha :-) "This is impermanent... oooh... soooo impermanent ... ahhh... ahhh ... and unsatisfactory ... hmmm ... sooo unsatisfactory *pump* *pump* --- NOT ME NOT ME NOT MEEEEE *hmph*" he he

(3) It might make you tired due to the depressive post-orgasmic effect, or restless if you don't orgasm. But this will be dependent on many other factors. E.g. during A&P there seems to be hardly any post-orgasm fatigue.
(4) It has been no real impediment to my progress on the path... that I can tell. Also, a large part of Yoga is simply a very complicated way of masturbating.
(5) Well, if it is (*straightening my back and putting on a very pompous tone*) "in the spirit of pragmatic dharma" (hi hi), then you'll just have to
uncover the truth behind (behind?) jerking off and enlightenment through the praxis of that very same dharma.
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Maybe you're giving this wanking thing too much importance? I mean, what do a few wanks matter?
Which is not to say or imply "don't masturbate on retreat," just simply that you can try both with and without, and see
what is best for your meditation. Do you get irritated and nervous if you don't act on your habit? Do you get tired and sleepy if you do? Is it indifferent? This will answer your question while giving proper importance to the important thing.
Remember that: every single moment of your retreat is
extremely precious, and that your number one priority while on retreat should be to make the very best use of your time.