This is the act of having a raging wank whilst in the shower.
It is done to be able to fit more activities into the day by combining two activties into one without having to go through the hassle of cleaning up after a wank.
Joan: whats up with you today, normally you very angry and tense?
Colin: well i've adopted this new programme into my life called shanking which applies a wank in the same time as i shower killing two birds with one stone and without the effort of not having to clean up after my intense wanking.