From the Elder scrolls wiki:

360 To perform a finishing move on command is done by tapping RT and moving the left stick forward at the same time. PS3 Performing a finishing move can happen randomly in a fight but can be done on command by tapping R1 and flicking the left analog stick forward. A finisher factors in how strong the player is against its enemy and how the player's character is positioned. PC To perform finishing move press forward button and Mouse button corresponding to the character's weapon-hand when enemy is at low health.

From the Elder Scrolls wikia:

360 To perform a finishing move on command is done by tapping RT and moving the left stick forward at the same time. PS3 Performing a finishing move can happen randomly in a fight but can be done on command by tapping R1 and flicking the left analog stick forward. A finisher factors in how strong the player is against its enemy and how the player's character is positioned. PC To perform finishing move press forward button and Mouse button corresponding to the character's weapon-hand when enemy is at low health.