The Ion Torpedo is a Star Card that basically functions as a "Missile Launcher" against vehicles.
The launcher itself takes time (around 2-4 seconds) to lock-on a certain vehicle. Its recommended to be behind cover to avoid getting surprised by enemies, since this can leave a player vulnerable due to going into first person and having the scope constrict vision. So one should be aware of their surroundings before attempting in-order to not be surprised.
The launcher is mainly used for anti-vehicle/turrets, on game modes such as Supremacy, Walker Assault, and Turning Point it is useful due to the amount of ground/air vehicles while being widely useless on Blast and Droid Run ect. due to being exclusively infantry focused. It is effective against enemy vehicles compared to other means and coupled with Ion Shot can make an effective anti-ground vehicle combo against AT-STs and turrets. It can lock onto air-vehicles, making it the only anti-air option in terms of star cards, but can be evaded by out-running (via Speed boost on Tie Fighters/Interceptors), Shields for Rebels or the Jammer for the T-47 Airspeeder, so its best to fire at basically the closest possible moment of the vehicle arriving.
When locking on, the enemy player will get a notification in the message of "Enemy Locking" in the middle of the screen, once fired the enemy will get a notification of "Missile Firing" with an arrow of where the missile is coming from, this can backfire if the player can pinpoint where the missile was fired and can counter-attack. Its recommended to change positions incase of such scenario. It can not lock on to infantry.