Using this will give an enemy monster of your choice the ability to summon a base copy of itself on your side of the board with +1/+1 when it dies. This effect will wear of once the monster has died (and gets resurrected) or if it gets silenced.
This is very useful, especially on cards like Frisk, who will stay in your possesion due to its infinite Dust, and The Heroine, who makes full use of itself due to not having a Magic effect. Keep in mind that the enemy will still keep their own Frisk or Heroine and that you simply clone them. It also works well with cards with simply high stats like Glyde or Asriel Dreemurr. And speaking of Dreemurr, you can also cast Will to Fight on cloned monsters with Magic afterwards to use their Magic later on.