Attacking a Called Target
Once someone calls a target, you may press the T key to immediately select the same target. You may then press the spacebar or click a skill in your skillbar to attack or use a skill on the called target.
Alternately, after pressing the T key, you may hold the CTRL button while pressing the spacebar or clicking a skill in your skill bar to announce that you are attacking the same called target. Doing so will post "I am attacking (target x)!" or "I am using (skill x) on (target y)!" in team chat.