Off-spin
Traditionally, finger spinners bowled with an emphasis on side spin. With the advent of covered and better batting pitches the focus changed to bowling with a component of top spin. The top spin provides loop which makes it harder for the batsman to make an early judgement of the length of the delivery. The ball tends to be released with a slightly upward trajectory which raises the ball above the batsman's eye line. The topspin on the ball then causes the ball to dip earlier and bounce higher.
The side spin component of the delivery causes it to drift, courtesy of Bernoulli's principle. For a stock, side spin or under cutter this causes the ball to drift away from the batsman. The combination of drift, loop and spin combine to provide the finger spinner's attacking ability.
Read more about this topic: Finger Spin