A guess, but if you fed 3 phase into an alternator (one without the diode pack) and energised the field winding, it would rotate wouldn't it?
Correct.
"The 787's engine-start and APU-start functions are performed by extensions of the method that has been successfully used for the APU in the Next-Generation 737 airplane family. In this method, the generators are run as synchronous starting motors with the starting process being controlled by start converters."
AERO - 787 No-Bleed Systems