His shikai, Tachikaze, is a wind-based zanpakutou that can launch long-range, string-like, blades that slices enemies. But what special abilities does Tekken Tachikaze, Kensei Muguruma's bankai, possess?
It was confirmed in chapter 560, taken from Bleach Wikia:
It appears to greatly augment his strength, as he is able to create a large explosion upon attacking. This is due to the fact that in this form the power of his Bankai is transferred to the knuckle blades. The bursting power continues to hit his opponent endlessly as long as his fists are touching his opponent.
According to Wikipedia:
Kensei's bankai is named Tekken Tachikaze (鐡拳・断地風?, lit. Iron Fist Earth-Severing Wind). The combat knife transforms into two bladed, bronze knuckle-like weapons known as tiger claws in each hand that are connected by a long fabric-like material that wraps around and up his arms like bandages and form a small arc above his head. Its abilities are unknown.