You have 3 questions:
- What exactly happens in the aircraft when disengaging the generators?
- What benefit does it have to be not able to engage them again?
- Is it only the B747 that works like that or also other Airliners?
What exactly happens in the aircraft when disengaging the generators?
As mentioned in the other answers:
- A solenoid removes a retention pin.
- This allows a spring-driven pawl to engage a worm gear on the drive input shaft.
- This causes the inner input shaft to disengage from the engine drive.
- Thus the generator drive rapidly coasts to a stop.
What benefit does it have to be not able to engage them again?
The primary reason to disconnect the generators is when some mechanical problem causes a risk of serious mechanical damage to them. From the Boeing 747-400 Abnormal Procedure ChecklistsWord Doc :
Condition: Low IDG oil pressure or high IDG oil temperature.
Affected GENERATOR DRIVE DISCONNECT switch PUSH.
Note: This action prevents damage to the IDG.
DRIVE DISC message is displayed.
ELECT GEN OFF message is displayed.
A lesser reason might be to prevent a seized-up generator drive from adversely affecting an otherwise operable engine.
Since whatever caused the loss of oil (pressure) and/or overheating almost never can be corrected in the air, then there is no point in being able to reengage the generator drives in the air.
Furthermore, to provide that capability would require complex mesh gears, clutches, and mechanisms that would add considerable weight and more points of failure -- all to cover a contingency that is both rare and very unlikely to be critical (discounting sabotage).
Is it only the B747 that works like that or also other Airliners?
Most large jet aircraft use some form of Constant speed drive (CSD) to power their electrical generators, as the alternatives are not well suited to the large electrical demands of jetliners.
From Wikipedia, we see that newer Boeing and Airbus models use a single-case variant of this called an "IDG".