Some of you probably have the Nexus 6P now, or are expecting it to show up soon. Unless you ordered a 128GB, that is. Sorry. For everyone else, there's now an official build of TWRP for the 6P. However, like the 5X, there's no support for data decryption yet.
TWRP is a custom recovery that lets you flash whatever you want to the device once you've unlocked the bootloader. You can install ROMs, backup your device, and flash random ZIP files that you probably shouldn't. If you break something, the 6P does have factory images that you can use to restore it to defaults.
This first release of TWRP doesn't support decrypting data, but the developers plan to add down the road. In the meantime, you can probably dig up a modified boot image to disable encryption.