Zappa got the best one.

If I read Dustloop wiki correctly ''IK'' stands for ''Instant Kill''.
Also in this game Oder-Sol & Dizzy don't get IK Modes. (For Dizzy her Instant Kill is just a super hard and long list of inputs and Oder-Sol have the whole Dragon install Combo think)

And it Seems Faust is the only one who just reuses animation from 1 of him Taunts, when everyone else get a fully special animations just for Instant Kill mode.