If I recall correctly pulling both engines back into flight idle against the stops is supposed to establish a 700 fpm descent....I believe this is tied in with a standardized descent rate for instrument approaches or the glideslope or something? (not being a pilot I'm not real sure about this).....went on a test hop one time when I was working for Atlantic Aviation in LGB
and when the guy eased the power back everything was fine, no splits, but when he chopped the power the difference in rpm decay dropped one wing in a BIG hole....good thing we had some altitude and were just testing stuff.
I worked on another MU2 one time, this had -6 engines on it, the pilot was stooging along all temps and pressures normal thinking life was wonderful, looking down at those BIG oil pressure gauges, and one engine chose that exact moment to lose 40 pounds of oil pressure out of a possible 60 pounds.....it took two trips inside the gearbox to find the trouble...there was a roller bearing high speed pinion mod that was done on the early marks and it involved modifying the gearcase and diaphragm and drilling three small holes in an oil gallery as peters for the gearset.
Teledyne down in Neosho, MO
had modified the diaphragm some 800 hours previously...busted a drill bit and scragged the three small holes,,,,pulled out the busted bit, patched the gallery with epoxy and tried again....it lasted 800 hours and failed just when the pilot happened to be scanning the panel....needless to say they forked over another diaphragm set without too much persuadin'.