It's no secret anymore, thanks to a website that gives a very brief description of the game.
3 modes will be available in the test. Ranked will pit you against an opponent with a similar player rating, and winning ranked matches will award experience and crates containing new cards. Challenge allows you to play a friend in an unranked match, while in the last mode, Arena, you'll queue up to face the current Arena champion, taking their place if you win.
We'll let you know more when further details are revealed.
Are you signing up to take part in the Technical Test? Let us know in the forums or comments below.