You now avoid the blows easilty. This is really annoying, especially for your enemy!
This greatly increases your chance to dodge. Weapons that normally are inaccurate, halberds and bumps come to mind readily, become a joke against a brute with this speciality.
Untouchable negates Implacable. Two brutes were pitted against each other, one with Implacable and one with Untouchable. The one with Implacable happened to roll bumps, a common mace-like weapon that has the worst accuracy in the game (other than the stone hammer; or "axe" as it's referred to).
The one with Implacable did not hit 100% like the specialty implies; due to the defending brute having Untouchable and Bumps being a highly innaccurate weapon to begin with.
While this seems inconclusive, not knowing whether the weapons innate innaccuracy caused the misses or if the defending brute's Untouchable did; it should remove the notion from any new player that getting a stone hammer and implacable makes them very strong. This is very rare though