A twin mass (or dual mass) flywheel is one made up of two components (discs) attached via a coupling which allows up to 15 degrees of difference in position of the two discs. Basically they are used to take up a bit of inertia when changing gear so that the load doesn't come back on with a 'snap' when changing up or down. This does mean that you lose a little power on acceleration and a little of the braking effect on deceleration but does give a smoother effect.