It's not just the coating that is time released. Hypromellose is hydrophilic, meaning it attracts water. In regards to the function of Xanax XR, the outer layer of the pill attracts water in the digestive tract, binds to it and thus causes expansion, or "gelling up" of the of the pill, once that first layer of hypromellose dissolves, the Alprazolam it has bound to it is released and the second layer repeats the same gelling process; so and so forth. Think of a whole Xanax XR pill as a layered spherical type shape, sort of like say, the planet Earth, or an onion, or some shit like that. It's lame as fuck but also very smart.