Powerblades are fitted to the forearm, enabling the wearer to use both hands freely. A well trained warrior can use sweeping strikes with the powerblades in addition to their other weapons. A model equipped with powerblades gets +1 Attack. This can be in addition to +1 Attack for being armed with two other close combat weapons for a total of +2 Attacks. A model with powerblades ignores armour saves.
Powerblades
Source: Codex: Harlequins, p. 12