What are the fundamentals of the counselling for admission for an MTech in IIT Roorkee (as they send call letters a to lot more students than are required?

Most branches conduct direct counseling without any official written test or interview. 

• Every department is assigned a fixed time slot for counseling and you have to attend the counseling of the department you have applied for.
• Candidates are summoned on the basis of their GATE score/rank
• First, students belonging to the SC/ST/OBC category are counseled. Those with a good rank are eligible to sit in General Category.
• Once the reserved category counseling has been wrapped, GC students are called.
• Anyone who is unable to get a seat can fill out a form for waitlisted candidates to be eligible for further rounds. Submit your DD along with it.
• Any vacancy will be filled with the first candidate on the waitlist on the basis of rank. Two rounds will be conducted similarly.
• Lastly, the Spot round is held in July to fill the remaining vacancies.

Here are a few points of caution one should consider regarding counseling

• Your branch preference cannot be altered once you have filled out the application form. For instance, if you chose VLSI>Communication and then want to opt for Communication despite getting a seat in VLSI, it won’t be possible.
• Assemble all necessary documents including DD and photographs in case you get a seat.

It’s important to note that not all students summoned for counseling get a seat.