This one's easy: if you use Android Pay on a Nexus device before December 31st, 2015, Google will add a $20 Best Buy e-gift card to your Android Pay... digital repository of cards. (Side note: what are we supposed to call the collection of cards and gift cards and loyalty stuff in Android Pay if not a wallet? This is awkward.) There is a limited supply of these gift cards, however, so the promo may end before 12/31 if people really get on this (I certainly will be taking advantage ASAP).
The nitty-gritty details aren't very rigorous, but here's what you need to know. You have to be using a Nexus 4, 5, 6, 7, 9, 5X, or 6P with Android Pay in order to be eligible. Starting today, Google will automatically earmark your account for a $20 Best Buy e-gift card on your next Android Pay purchase. Obviously, this is limited to one gift card per account. Non-Nexus devices are not eligible.
So, get out there and start tapping! I'm heading to my local Trader Joe's for groceries this evening, I'll tell you that much.