They pre-purchase annual oyster cards and sell them back to you at a weekly rate. They recently got Apple Pay which is really useful with the express travel option so you don't need to use touch/face id to tap in.
They still have consistent problems with randomly charging you random amounts on top of the weekly cost. It's a normal MasterCard so you can check it on the TfL website, but I've had to work with them a few times to get refunded from random extra charges (usually 0.50 to sometimes 7 quid) which they can't explain why they happen.
I don’t know how they undercut TfL on the price but I presume they’re doing something tricky with redistributing monthly or annual passes.