Too easy - just wind it back (vernier throttle in the Bo, in my case!) until the ETA at the waypoint on the GPS is the same as the ATC required crossing time!
Even I can do it, so it must be real easy.
Eeeeerrrrrr ---- Aaaaas long as you don't fall ( or "plummet", the media preferred technical tern) out of the sky doing it, as did a BA 747 over Turkey a little while back !!!!! True story, all graphically witnessed by the QF1, about 4nm behind, and 4000 lower.
Times when I wish I had had a camera, No.5791.
What is known in the trade as having a "Nigel" as an F/O, when the Captain is having a quick leak.
Tootle pep!!