If you'd like to be part of the beta, you can visit https://manage.stripe.com/faster-us-transfers and if you're US-based, you'll automatically be registered to participate in the beta. We're incrementally adding users to the beta, so we'll let you know as soon as you've been added.
(I work at Stripe.)
What I'm wondering is, "how is this possible?" How is Stripe now moving the money faster than before? What change made this possible? Did the stewards of the ACH system make a change that allows this? Is Stripe advancing funds ahead of time based on some kind of credit? Was the seven-day policy basically arbitrary before, and so changing it to two-day basically involves changing an integer somewhere in the codebase and updating documentation? Or is it something else happening at a different layer? More generally, what are the pieces at play in transferring money that influence transfer time? And which of these pieces has Stripe interacted with in order to transfer funds faster?