These days medium pace and spin bowling are two distinct schools, one incompatible with the other. But there has never been a more successful style of bowling than fast-medium spin, as purveyed by Barnes in particular. He was, John Arlott wrote, "a right-arm fast-medium bowler with the accuracy, spin and resource of a slow bowler". Barnes, "square shouldered as a tailor's model" as Alan Ross put it in his poem, is said to be by men who saw them both to have been around the same speed as Alec Bedser, which suggests he was bowling between 70 and 80mph. These days Swann is reckoned to bowl quickly for a spinner, and his average speed is around 60mph. Barnes's stock delivery was a fast leg break that swerved one way in the air and then span back the other off the pitch. He married this with a fast off break that did the exact reverse, a ball he was taught by the Australian Monty Noble, another early master of spin-swerve bowling. Barnes's particular release meant that the two were difficult to distinguish. He did not unfurl the wrist for his leg break, but rather ****ed it backwards and rotated it, as though he was, as Rajan says, "unscrewing a light bulb". If you want a more technical explanation, you can find one in Bob Woolmer's Art and Science of Cricket.
This method brought Barnes 189 Test wickets at 16.43 each, and universal recognition from his contemporaries as the greatest bowler of his era. It was the swerve that did it, movement akin to the drift you still see now in good spin bowling, only faster through the air. But this was genuine spin bowling – Barnes was outraged when David Frith once had the temerity to ask him if he cut the ball? "'Cut it?' He glared, and again I wondered if he might hurl something at me. 'I spun the ball!'" The great Australian batsmen Clem Hill remembered how a "ball pitched outside my leg-stump, safe to the push off my pads, I thought. Before I could 'pick up' my bat, my off-stump was knocked silly".
A few bowlers have dabbled in something similar since, notably Derek Underwood, but Barnes's is a type of bowling that is not used any more, though. Why is this? Partly it must be the pitches, which are so much better to bat on than they once were. Covers mean that there are no sticky dogs any more, and improved drainage and groundskeeping ensure that there will always be less help for the bowler. Partly it must be the coaching – Barnes only had three hours of the stuff in his entire life – which would frown on such unorthodoxies, pigeon-holing players into distinct categories and persuading them against bowling at medium-pace.