While waiting I check the engine oil level and tires, and bolts, and chain tension.
Later I removed the back wheel, refitted it, adjusted the chain tension and replaced a headlight bulb.
When a bicyclist pedals harder, the added chain tension forces the arm to shift through the rear gears.
This had been attributed to loss of chain tension.
Any instrumental way of reporting the peak chain tension to the truck's driver, even roughly, would be helpfull to accumulated experience about this.
This may require unbolting when the wheel is removed or moved in its fork ends to adjust chain tension.
One method, used by Shimano, is to use chain tension to pivot the cage.
Higher chain tension was found to be more efficient: "This is actually not in the direction you'd expect, based simply on friction".
Bicycles with vertical dropouts and no derailleur require some way to adjust chain tension.
Single-speed bicycle builders have devised a variety of methods for producing and maintaining the proper chain tension.