Some person finally
put this on the web. JVC published a number techical guides over the years describing new features on their VHS lineup. This doc was referenced in the later
one that covered the 1998 models (HR-Sx500 and so on) and some service manuals, but I had no luck finding it published anywhere.
This doc describes the inner workings of the dynamic drum system, specifically for the pre-1998 models, as they changed the mechanism a fair bit for that lineup (after which there were only minor modifications untill the very late-model decks like the sr-vs30u which used a substantially different mech). The 1998 guide has a whole section on the mech, but doesn't note anything about the dynamic drum system so I presume they didn't alter much of the core principles of it.
As has been subject to a bit of discussion in the past, the doc notes that it is only used for "trick play", and is not active during normal playback. It also notes how it's initialized electronically, which could maybe give some clues to how to disable it without doing to much manual tinkering, tho as mentioned they changed the mech a bit for the later models that we're mainly interested in.
The BEST/Video calibration function is also covered, the playback part seems to be a bit more simple than I thought it would be, altering some EQ parameter (not entirely clear if it's on the rf signal or the demodulated video) if the video signal frequency spectrum deviates from some pre-set default. It's not impossible that they made it more complex later though, but I don't know.
Attaching both here in case they disappear from the original location.