When picked Heat-Seeker Card, it will activate heat-seek function in Rocket Launcher for the next 5 rockets. The red glowing notches on the launcher shows how many heat-seeking rockets are left. The rockets can be shot not all at once and saved for the later moment by selecting other weapon. Once all 5 heat-seek rockets are shot, Lo Wang will need to find another card to activate the function again.
The heat-seeking function is useful when wanting to take out moving targets from a far like bosses.