The bowler delivers the ball with the same finger action as a normal off break but cocks the wrist so that the back of the hand faces the batsman. This gives the ball spin in the opposite direction to that for an off break, causing it to spin from the leg side to the off side to a right-handed batsman.
The doosra is the off-spinner's equivalent of the leg-spinner's googly, which spins in the opposite direction to the leg spinner's stock ball.
It is possible for a left-armer (whose action mirrors that of an off-spinner) to bowl the doosra, which in this case would turn from off to leg. Sri Lankan left-armer Rangana Herath gained recognition by bowling the delivery, in particular against the Australians during an A tour. England left-armer Monty Panesar has claimed to have bowled the delivery occasionally in domestic matches.