Spank the Plank means to play the guitar. Particularly in reference to "Rock and Roll".
To spank is to hit the bottom (normally of a child) repeatedly with the open palm of the hand as a form of punishment.
A plank is a long flat piece of wood used for carpentry and in the making of floors and wooden boats.
If we imagine a person spanking a plank it conjures up a vivid metaphor of a rock and roller playing the guitar.
For example: I would like to spank the plank like Jimmi Hendrix or Eric Clapton.