rudderrudderrat: Furst Ferret is correct! It was a procedure designed to help maintainence rig the nosewheel steering correctly. IIRC trim could be applied when taxying in a straight line, up to a maximum amount and this was noted for the engineers.
The procedure called for the trim to be zeroed prior to take-off. There were limits on how much left/right trim could be used and still considered normal.