It's an analog filter that offers the ability to tweak parameters like resonance in real-time or, from what I understand, in automated sequence. With the resonance of a filter changing, you get a very unique, sharp, buzzy sound that is immediately associated with synthesizers. A pretty revolutionary idea for guitars, really.

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Am Std. Strat (Lace Sensors Gold/Silver/Blue, GFS Delay Mod Board) -> Pignose Detonator > Ibanez TS7 (TS9 setting) > Ibanez TS7 (Hot setting) > Rocktron Vertigo > Danelectro Reel Echo > Vox Valvetronix AD50VT