Honesty, I was thinking putting the "what are taters master" giphy right here, as I had to Google about fowfers.
Overall, what I read about it, I certainly wouldn't do it while sleeping or close to sleep.
With certain exercises it's really essential to be awake and to monitor how your penis is feeling.
You don't want it to drop off now, do you? :ROF
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I can't answer about the gaining part. I haven't done fowfers. I have read about certain exercises utilizing your body weight in and those I have done only few times, nothing in terms of a routine.
Passive gaining without devices, to me it seems like utopia ... Of course resting after sets is kind of "passive gaining", as you need to rest to grow.
Why not be more active and make use of short pauses etc., piss pulls and whatnot.
I for one enjoy my sessions, be it by hands only work or with a device.
I'm passive as it is anyway, better be active in some sense.