Through the screwdriver shank, as it's placed across the capacitor terminals. This creates a dead short through which the differing potentials on the capacitor plates can equalize - and if the capacitor is large enough, they do so quite enthusiastically.

I ruined a screwdriver of my own that way once, discharging a photoflash cap in a flash head whose control circuit had died with the cap at full charge. Didn't do my hearing any good, either, I'm sure - it took fully half an hour for the ringing to go away.