I got a flash for my DSLR camera: A
mecablitz 52 AF-1 digital Canon with multi-language manual from here.
In section 15.2 (page 187 for english), it states:
15.2 Conditioning the flash capacitor
The flash capacitor built into the flash unit undergoes a physical change when the device has not been used for a long time. For this reason it is necessary to switch the device every three months for approx. 10 mins. The power supplies must deliver enough power so that flash standby lights up no later than 1 min after switching on.
I'm wondering: What physical change could a capacitor undergo, which can be more or less avoided by charging it every three months?