The number of speeds the system will shift for, and the distance that it will move per shift, is controlled at the rear derailleur.
In other words:
6770 can shift 11speed, if you use a 9070 rear derailleur. 9070 can shift 10speed, if you use a 6770 rear derailleur.
This is how the Shimano guys at interbike explained it on more than one occasion.
Working with very small electric motors (amongst other things), I assume that means the motors are not 'stepping' motors, but infinitely variable - otherwise you couldn't have a 10-step motor run 11 cogs
Well, along with the 9 (10spd) or 10 (11spd) steps for gear change, they also have 15 steps inward and 15 steps outward from the initial position.
I thought a stepper motor was configured such that 'X' number of steps were assigned to Rad or 360 degrees, not equated to the output of a linkage system (ex. the derailleur). Meaning the stepper motors are fabricated such that there could very well be 500 steps (the ones I've used had over 2000) per 360 degrees of rotation. So to when programming how many steps would equate from one cog to another would be something like 1500 or 2000 steps, depending on if there is a gear box, or other mechanical advantage linkage (i.e. RD).
If this was the case, someone could easily reprogram the # of steps to go from 10 to 11 cassette. Someone tell me if I am way off on this. FYI, I still use cable shifting. The Di2 is still voodoo to me.