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.

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.