How does he do it? A couple of things:
1. He doesn’t have a stock delivery. Some take that as a weakness; others think its an inherent part of his guile and difficulty. So, he bowls googlies when off-spinners usually don’t (wristy leg-spinners prefer that); he also bowls quicker, which gives batsmen less time to adjust to his various spins. This is a crucial factor of his success: unlike Murali, who often relies on loop and flight, Mendis goes straight in.
2. But there’s also the fact that he bowls differently. There’s the infamous “carrom ball,” which is explained fully here. I’ve included some videos below; watch as the middle finger just flicks the ball, which then does as Mendis pleases, spinning into and away when he wants.
All this does not mean that the Sri Lankan spin duo is invincible: looking at the batting highlights, India should be embarrassed, playing without purpose or restraint. It’s back to good ol’ fashioned Test cricket now. Move the feet, play late, and keep your eye on the ball, boys.
And finally, Mendis’s first Test wicket: