Carlos Marmol signed a three-year contract on Monday. It may be a dangerous contract for the Cubs, as Marmol walks a ton of batters. He gets away with it, however, due to his very high strikeout rate. In 2010, he struck out 16.0 batters per nine innings while walking six per nine. He only allowed 40 hits, however, and without the hits, the batters who walk can't advance very far. Keeping the ball out of play kept runners stranded on base.
Marmol works against batters with two very different pitches. His 94 MPH stays up in the strike zone with very little drop. That probably helps keep his home run rate low. His 84 MPH slider drops a great deal, and moves to the right hand of the catcher (down and in on left-handed batters, away from right-handed batters).
Marmol uses the slider as his out pitch. Early in the count he tends to use the fastball, as you can see by the movement of the pitches:
He does mix in some sliders to keep the batters honest, and with one strike goes to the pitch even more.
It's about a 60-40 split, with the fast ball getting the nod trying for the first strike, the slider dominating the second strike. For strike three, it's almost all sliders.
With two strikes, Marmol throws 80% sliders. With 75 of his Ks coming on the pitch. It's a great example of a pitcher moving a batter's eye, not letting him get comfortable with a pitch or a location.