What is the reason of Magnetic Braking in contact binary stars?
Just dark spots?
I mean hot spots can have this effect on binaries.
The magnetic braking in contact binary stars AFAIK is assumed to be similar to the magnetic braking mechanism that spins down the rotation of single stars.
A stellar wind couples to the large scale magnetic field and maintains this coupling out to the Alfven radius (where the thermal pressure and magnetic pressure are roughly equal). At this point the wind decouples, stops co-rotating with the star and hence removes a certain amount of specific angular momentum.
The situation will be more complicated in a binary system (entangled, asymmetric fields etc.) but the basic principle will be the same. Many contact binaries have fast rotation and strong, dynamo-generated magnetic fields.