Basically, it consist on grabbing your firehose while seeing porn on the internet... sometimes causing you to use ur left hand to move the mouse around cause you are stroking usin' the right hand (if you are left handed, it causes you to use right hand to drive your mouse)
To do GYDADC in centain places such as internet cafes, friend's houses or at school, represents a high risk only pros can avoid (needless to say its better not to sake hands with a GYDADC pro).
Notice down at the example how can GYDADC be used as a verb.
>Grab ur dick and double click 4 PORRN PORRN PORRRRRRN!
>Hey doid! it sux in here! did ya just GYDADC'ed??
>LOL! yu got me doid!