In its simplest terms, the act of a tommy tank while the subject is simultaneously running.
Whilst seemingly simple, the true execution of this feat has become a furiously debated subject in circles of dedicated practitioners of the art. It is believed that the study that led to Werner Heisenberg's Principle of Uncertainty was inspired by his consideration of this question.
The reason for the surrounding controversy is not the question of whether either of the feats are possible, but whether they are in fact truly simultaneously possible.
The successful achievement of a running wank involves the performance of the act in its entirety, to its culmination, i.e. from inception of wood, through the vinegar phase, and to the documentable release of sex wee, while continuing the running motion throughout. Herein lies the debate - it is believed by many that this is in fact impossible, as it is argued that successful completion of the former must lead to an involuntary buckling of the knees, and therefore voiding the condition of continued running.
To date, despite claims from either camp, including various unconfirmed reports of the act being witnessed, no satisfactory answer has been provided. Several great minds throughout history, such as Erwin Schrödinger whose famous quantum expirement was inspired by the question of determining whether the achievement of spaffage, regardless of any ensuing fall can be said to have actually occurred whilst in locomotion - the runwanker is at this moment in time in the undeterminable quantum state of both running and falling.
How the fuck would I know if I can do that? You may as well ask me if I can have a runwank.