You have a website http://master.com and want to have separate domain for mobile version, i.e. http://mobile.master.com.

Slave site mobile.master.com should be created as parked domain for master site in your hosting panel first.

Next slave site is configured in MightySites and default template style is assigned, so slave site will display same contents as master site but use own default template style (i.e. mobile responsive template).

All contents are managed from backend of master site as usual.

Each site can use own template style and load own CSS file, certain menu items and modules can be hidden for any site.

MightySites doesn't automatically redirect user from usual site to mobile site by detecting a mobile device.