I'm guessing it was something like a jumper over an AGC (auto gain control) circuit VCRs use to avoid saturating the tape, since the MacroVision mechanism was b

I'm guessing it was something like a jumper over an AGC (auto gain control) circuit VCRs use to avoid saturating the tape, since the MacroVision mechanism was basically to intermittently blast out-of-frame lines with too-intense brightness, which has no effect on a typical TV, but does make the AGC clamp down so the whole frame is intermittently too dark.