To commit tactical suicide, oft in FPS games. Tactical suicides result in tactical superiority, whether by positioning the player committing it into a more favorable location, filling up his health, or denying the enemy a kill / attached bonuses.
Capture the Flag: You're on defense, low on health. You can see the incoming enemies half way across the map. Yet, there are no health packs around. TAC and respawn with full health, ready to put up a more efficient defense.
Deathmatch: Score limit is set to 20; the enemy has 19, and has you cornered. A frag, and his victory, is inevitable - unless you TAC.