BUTTERFLY TENERGY 05 HARD. The latest member of the popular Tenergy family is now offered with a much harder version of ‘Spring Sponge’. With a sponge density of 43 degrees, Tenergy 05 Hard is great for generating powerful and precise counter topspins as the hard sponge responds to a stronger impact enabling more power. Players with a well-developed technique will feel the benefit. European Champion and Butterfly Super Star Timo Boll is using it already!