If you want to understand leg spin, you can do worse things than buy the excellent album 'The Duckworth Lewis Method' by the band of the same name. 'Jiggery Pokery' is one of the few cricket songs I'm aware of (indeed, aside from 'Dreadlock Holiday' by 10CC and the rest of the songs on the DLM album, I can't think of any) and the only one that sets to music 'The Ball of the Century'. But more on this later.
Leg spin is a type of wrist spin. Rather than bowling the ball out of the front of the hand, imparting spin by a rapid twist of the wrist and fingers, wrist spin is delivered from the back of the hand, with the rotation of the wrist and shoulder imparting spin on the ball. Try it with a tennis or ping pong ball. Hold the ball between your fingers, palm facing in the direction you want the ball to travel. Then twist wrist rapidly and as you do so flip the fingers forward, so that the ball shoots out of the back of your hand. It probably won't go where you expected it to, but if you do it right the motion of the wrist should impact spin on the ball. And, hopefully, you won't strain anything.
So here's Terry Jenner, the mentor of a certain great legspinner, explaining how it works.
There are three big advantages to leg spin. First, the stock ball (that is the ordinary leg spin ball, the one the bowler bowls most often) turns from leg to off (the opposite way to the stock ball of the off spinner), which means it is spinning away from a right-handed batsman (in the same way as an SLA). Second, if you do it right, wrist spin imparts more spin on the ball than finger spin. So you can get more turn. (It also imparts more strain on the body - more than one leggie has had to have shoulder surgery). And third, by varying the position of the wrist at the point of delivery, it's possible to bowl a googly.
Bosanquet was an Englishman, and it's entirely possible that no Englishman, not even Grace, had more of an impact on cricket. For 'Bosie' invented the googly. He invented it experimenting with a ping-pong ball. And then he transferred it to the cricket pitch. Suddenly it was possible for a spinner to bowl a ball that turned the 'other' way. For a right-arm wrist spinner (leg spinner) this meant the ball would turn from off to leg, rather than leg to off. And if the batsman wasn't expecting the googly, he was going to end up looking like an awful chump.
Leg spin was big between the wars, when players like Faulkner, Grimmett and O'Reilly mastered Bosie's googly and tormented (mainly English) batsmen. For some reason, probably because despite all the advantages of leg spin it has one major disadvantage - it's so damm difficult to master - leg spin fell out of fashion. There is nothing worse (for a fielding captain) than a bowler bowling poor legspin. Ian Salisbury demonstrated this once or twice.
But Abdur Qadir revitalised the art in the 80s, and then Shane Warne burst onto the scene. People who'd never watched cricket in their lives would have been aware of Warne. He was the biggest thing to hit Ashes cricket since Botham. And what an arrival.
I mean, watch that again. The ball lands a long way outside leg stump and hits off! It hits off! Gatting plays a perfect forward defensive to a leg break and is clean bowled. Incredible. No wonder they call it the 'ball of the century'. It was the first ball Warne had ever bowled in Test cricket in England. That ball, and Warne's absolute mastery of the art, cast such a spell over England batsmen that for years after, even when they couldn't lose, they did.
In the end Warne ended up with 708 Test wickets. Murali got 800 (his 800th was the last wicket to fall in his last ever Test - some timing!). The debate about who was the greater bowler of the two rumbles on, but it's hard to argue with the assertion that those two were the best of all time. Then again, maybe Bill O'Reilly would have taken 800 wickets if he'd got the chance to play 145 Tests!
Final word on the subject of leg spin goes to Terry Jenner, who helped Warne develop into the bowler he became. Here Jenner explains the five different balls(!) that made up Warne's armoury.