When performed, Cell pumps himself up like a balloon and explodes after one minute. The resultant explosion is capable of destroying an entire planet. If Cell is attacked in any way before the countdown reaches zero, he will explode immediately. Once activated, Cell cannot reverse the technique.
Cell uses Unforgivable! in an attempt to blow up the Earth to defeat Super Saiyan 2 Gohan during the Cell Games. However, Goku prevents him from doing so by taking Cell to King Kai's planet via Instant Transmission, and Cell ends up destroying this planet instead, killing Goku, King Kai, Bubbles, and Gregory. A small part of Cell survives the explosion, and he regenerates and becomes even stronger than before, reaching his Super Perfect form.
Appearances in games
Unforgivable! was named in the video game Dragon Ball Z: Budokai Tenkaichi 3; in the game, he rushes at his opponent, then says "I won't allow this, YOU WON'T GET AWAY!", pumps up, and self-destructs, damaging himself and his opponent. The technique is called Self-Destruct in Dragon Ball Z: Supersonic Warriors 2. Though the move itself never appears in the game Dragon Ball: Xenoverse, Cell almost uses it to try to get a reaction out of the Future Warrior after they complete one of his training missions (implying that the technique can be used in his Perfect Form).