The player has autofire. No buttons need to be pressed. The player emits a green laser pulse automatically. As soon as one laser pulse disappears (either due to going off the canvas or due to hitting a ship or canon), the next laser pulse is emitted automatically.
For that reason, you may notice that the laser pulses appear more frequently while hitting low flying ships because it is quicker to hit a low flying ship than a high flying ship.
Does the player's ship have autofire or is the player supposed to use a fire button?