Known as Torsen - Torque Sensing. Using planetry gears to send more torque to the wheel that has grip, as opposed to friction plates that 'lock' to differing pre-determined amounts.
A slippy diff of some sort can be wonderful for helping to address traction out of bends when you would otherwise just spin power away through the inside wheel, but its a lot of money and can have some disadvantages if you are just driving on the road. For day to day driving on the road, the traction control system on the Leon is perfectly adequate (despite the hero's that think turning it off as soon as they get in the car will make them miles faster).