The IObanez FL9 Flanger is an analog flanger. Flange is a type of phase effect that produces a wide range of "swooshing" effects on your guitar tone.
It has your standard 1/4" inputs and outputs, AC adapter input, and red led, all in a sturdy duck yellow metal box.

Flange is a complicated effect. If you've never used one, understanding what the 4 knobs control takes a bit of experimenting. The FL9 has knobs for "delay time", "regeneration", "speed", and "width", and can be a bit daunting to master. Sometimes I create a cool sound, and then forget how to achieve it again, the tange of sounds possible and level of control is probably unmatched by any other stompbox flanger on the market. But for a devoted knob twiddler, you will be in heaven. This box is not capable of making a "bad" sound, only strange ones you may find unusable at the furthest extremes of its spectrum.

The FL9 is classic analog flange sound all the way. There is no harshness, nor thin-ness. It is a thick and warm flange, that will make your guitar warble and whoosh for days. Add in an analog delay in your signal chain, and you'll be in space cadet heaven.
I especially like the ability to dial in some Andy Summer's Police sounds, which are on the more mild side, all the way to extreme whoosh effects that make the room feel like its spinning and are unusable except just to sound wicked.