I've seen a few examples of actors rehearsing a scene, but in different accents to their own.
In Shaun of the Dead, Simon Pegg and Peter Serafinowicz pretend to be John Lennon and Paul McCartney.
In Star Trek, Chris Pine and Zachary Quinto rehearse in a German and Irish accent, respectively:
And Michael J Fox in Back to the Future:
I imagine it's a way to let off steam, or make a long day of filming entertaining, but is there a specific term for what's seen above?
For example, does the director say: "One more take, but this time let's do [term]", or is it more likely an idea that springs up on set?