The Lightning power-up is in the bottom corner of the match-3 screen. It’s chargeable: every time you detonate a regular power-up, the Lightning progress bar charges a little more. Exploding several power-ups at the same time adds extra points to your charge.
Once the power-up is fully charged, a Lightning is randomly placed on the game field. Swap it with any adjacent piece to remove all the pieces of that color from the field!