This is when you masturbate in an unusual way.
You could be upside down at the time, using two elbows, your feet, sports equipment or rolling pins.
The purpose of extreme wanking is to spice up your wank life
Extreme Wanking example
The Andre Agassi : Double back hander
1. Assume a seated position
2. Point your legs to the heavens.
3. Put your hands underneath your legs.
4. Put both hands around your penis.