Prepatches are the equivalent to alpha releases for Linux; they live in the
testing directories in the archives. They should be applied using the
patch(1) utility to the source code of the previous full release with a 3-part version number (for example, the 2.6.12-rc4 prepatch should be applied to the 2.6.11 kernel sources, not, for example, 18.104.22.168.)
testing/incr directories contain automatically generated patches from one prepatch to another.
Prepatches may be poorly tested, and may in fact not work at all. Use on your own risk; if you use one and run into a problem please see Reporting Linux Kernel Bugs on the front page.
Prepatches with -rc in the name are considered release candidates and may become full versions. It is therefore particularly important that those get tested.