Synching
Of course, 'twas only the copilots who needed the "little propellors" to synch those mighty Speys. Most Captains had been educated on the Varsity (two Bristol Hercules Radials) then the Shack (four R-R Griffon V12s) so they could synch four Speys by ear; much easier and much faster.
The Nimrod (Ah! de Havilland!) crews had little patience with pilots who were tone deaf.