Otto

[Lemminkäinen]

Permanent skills with a cost need to be activated but unlike Action skills, they can be activated at any time (Actions skills can only be used during the mini's own Activation). Permanent skills with a cost define how long they are in effect once activated.

Otto's English card says:
"Chain saw (Permanent) (2 SP): The next time Otto performs a melee attack, each success of his CBT roll (after relocation and dodges) deals two wounds instead of one."

So you can pay the 2 SP any time you want (e.g., when someone attacks you and you respond) and it is in effect in Otto's next melee attack.