I LOVE this stuff! If you leave it on for the full 30 minutes you can get up to two levels of lift, but most people use it as a base breaker after highlighting to slightly lift the natural base to soften the contrast between the natural hair and the highlights. You can also use it just around the hairline if you want to lift those baby hairs that are impossible to foil, or for clients that want to brighten their roots in between highlighting services.
It's ammonia free and lightens the hair really softly so you don't have to worry about the hair suddenly turning too warm. It will of course make the natural colour slightly warmer, but it's not like a raw undercoat like when you lift with a tint. Just watch it and rinse before it becomes too warm. If you are only lifting 1/2 shade or 1 shade lighter it's not really a problem (unless the client hates any warmth). If you leave it on longer it will expose more warmth, so you just have to eyeball it.
The way I describe the effect to clients is that it lightens you hair the same way as if you had been out in the sun, naturally brighter. So some people have used it to create soft ombre effects leaving it on the ends longer and gradually working it up the hair. You can mix it with 20 or 30 vol, on dry hair or like a shampoo at the basin (it is very runny). The best thing is you DON'T have to skin test! So it's an easy way to up sell.