Like you sweep your table with a piece of cloth - Batsman facing spinners, sometimes, meet the ball with their bat as sweeping object & paddle it down the corner on leg side to make a run.. this is called Sweep shot.
History suggests Graham Gooch of England first started Sweep shot against India in a World Cup match at Mumbai in 1987. Gooch hit 115 that day..
As cricket evolved, reverse sweep shot came into picture were batters changed their grip and swept the ball to off side where fielders were less. Andy Flower of Zimbabwe really mastered the art of reverse sweep shot but now - Kevin Pietersen of England has come up with another mode of sweep shot where he jumps & changes grip as lefty and slogs the ball in full throttle!
In fact, it has to be called switch shot where right handed batsman switches his stance to that of a left hander and vice verse. nothing sweeping about it.
As the fielding was set for a right hander, Kevin Pietersen hit 2 sixes as left hander that day against New Zealand in June 2008.
Is this legal? certainly not illegal though, and ICC has gone into record to discuss the matter and install some changes to rules of the game of cricket so that wides and leg-before (lbw) decisions can also be changed once the batsman plays this 'switch hitting' shot. This shot is also not illegal cause it opens up good chance for a bowler to get the batsman out. The only bad aspect is that a bowler has to inform the umpire and the batsman as "right arm over the wicket" or "left arm around the wicket" but no such prior notice from batsman that he's going to play a switch shot.