How is the drain of M1 locked at Vref?

 

How is the drain of M1 locked at Vref?

Do you mean Vtune?
The feedback forces the two inputs of the op amp to be equal. If you don't understand op amps and feedback, read this: http://www.allaboutcircuits.com/vol_3/chpt_8/4.html

 

But in this diagram we have positive feedback, a silly mistake ?

 

But in this diagram we have positive feedback, a silly mistake ?

You're right. I missed that.
Probably a mistake, as you said.