Open it in mobile Safari or Chrome and you'll find the boards already work — drag a card, drop a comment, check in from wherever you are.
You'll get to your boards from the couch, the airport, or the back of a taxi — the whole app fits on a small screen, and the touch targets are sized for thumbs, not styluses.
Your phone buzzes when someone @mentions you or a card you own moves — nothing else, unless you ask for it.
Buttons sized for fingertips, menus that don't fight you, and drag-and-drop that actually works on a touchscreen.
Sign up, open the link on your phone, and you're in — the native apps are on the way for when you want them.