Most likely aerodynamically relieving trim wheel pressure by intentionally letting the nose of the plane drift down. This allows the pilots to more easily crank the trim wheel and fix the issue.

If I remember correctly, this procedure was specifically described in older 737 manuals, but had been dropped more recently.

It's a "roller coaster" procedure where the plane is pitched down to lessen aerodynamic forces on the stabiliser, allowing the trim wheels to be manually turned, then pitched up to regain altitude. The process is repeated as necessary until the trim wheels can be manually moved without it.

Interesting, it seems similar to how large sailboats need to head into or down wind when attempting to trim sails in some conditions? (to reduce pressure on the sails to allow trimming to be doable by hand)

Yeah, with the important difference that in sailing you lose speed (and heel) when heading into the wind, whereas a plane will go even faster towards the ground... Scary!

Yes. Mentour mentions it in the video I have posted above.

You unload the stabilizer by pitching down for a few seconds so that you can trim it back somewhat.

However, that is not something you can do close to the ground (you lose altitude, obviously) and the "vomit comet" ride won't be appreciated by the passengers neither.

Basically in this situation, even though the nose of the plane is uncontrollably pointing down, to get it to rise again requires that you momentarily push the nose down even further to allow the trim control to unjam and start working again.

If I remember correctly, this is a procedure that once was on the 737 flight manual as an recommended procedure but later Boeing removed it. I recall reading about in an article here at HN.