Pre pigmentation is the process of depositing tones that are usually found on said base level, which are removed by lightening the hair.
By looking at the lightening curve I've attached you will see what you should have underlying on each base level. So if your client was a base 10 and you wanted to take her down to a 5 you will need to pre pigment the hair with red - orange. Pre pigmentation is usually done with either pigmented mousse. E.g.(indola colour mousse), or wella perfecton. There's probably plenty of other products that you can use too. In a nutshell you're putting back in what's been taken out. I hope this helps.