The short, rounded wings allow him to snake through tight places and to make quick turns. The long tale serves as a rudder. This alone however does not make him a match for a healthy chickadee or titmouse. The element of surprise gives the hawk a couple of seconds advantage. He still misses quite often though.
Studies show that sick and crippled birds are the ones most often caught by accipters.
Big flocks of birds concentrated at feeders are an unnatural occurrence in the natural world. In these gatherings disease can spread quickly. By culling the sick birds, Sharp-shinned Hawks help to prevent this. I have noticed that when the birds at my feeders sense danger their first reaction is to freeze. Sometimes its a passing cat or dog.