Clock based games are split between a decision phase and an execution phase. In the decision phase, the game is paused, and players are given time to dictate their next action. Once player actions are issued, all player actions are performed simultaneously using the in-game clock. Some actions might take longer than others. Whichever players action finishes first is allowed to issue new commands immediately, effectively giving that player an earlier turn.
Wikipedia—Turns, Rounds, and Time-keeping Systems in Games