Manual reversion
That particular check is a test to see whether the balanced anti-balance (servo) tab on one of the elevators is doing its job correctly. This could very well be what they were doing as it is part of the Boeing approved acceptance test curriculum. The reason this tab is there is to provide some alleviation of the control forces in case of a total hydraulic failure.
If the linkage connecting the tab to the elevator and horizontal stabilizer is adjusted wrongly it can potentially result in a violent pitch-up or pitch-down when the hydraulics fail or are manually switched off, because the tab will start to move the elevator to an aerodynamically neutral position, much like a trim tab on smaller aircraft, except of course for the fact that this one can't be adjusted in flight. Where this neutral position is relative to the stabilizer, is defined by how the adjustable linkage is set on the ground. This job requires some extremely precise work.