I read in the Elec systems chapter that if we press and hold the IDG pushbutton for more than 3 secs it’ll damage the disconnection mechanism. What exactly happens? Also it says that not to disconnect it if the engine is not running as if we do and then start the engine it’ll damage the IDG. How does this happen?
Pressing the IDG disconnect sends a current to a solenoid. A solenoid is "a cylindrical coil of wire acting as a magnet when carrying electric current."
When activated it pulls a pin that is holding another spring loaded pin that engages the disconnect mechanism.
Holding it for +3 seconds will overheat and thus destroy the solenoid and will require replacement to be able to use it again.
The disconnect mechanism requires a running engine. See here: What happens when disengaging the electricity generators on a B747?
So if it is not running, the next start may also damage the disconnecting mechanism.