suppose we're talking about NIT W. let NIT W have 2vacancies in ECE, 5 in civil and 11 in bio tech. so they'll publish that in all they have 18seats vacant. when students apply, on the basis of their rank they will be allotted "NIT W without branch" so you have an institute but no idea about the branch. then they'll conduct internal counselling by which students who were already in NIT W can upgrade their branch eg. students of EEE can slide to ECE.. now EEE will have 2vanacies filled by mech.. and so on till all the vacancies are only in the lowest branch. then the newly admitted 18students will be allotted the lowest branch