Butterfly Boll Control Proline w/Rozena, in the Boll series, the slowest of the four blades, but offers the highest level of control. Rozena offers exceptional speed and great spin! Rozena is powered by Butterfly's Spring Sponge technology. Spring Sponge is used in the popular Tenergy rubber series which is used by many world class players. The softer sponge layer is a bit more forgiving on errant shots. Spring Sponge makes it easier to play your shots and the tremendous feel of Spring Sponge must be experienced to believe.