It is given in the Brochure ki when u report at the admission centre after 1st counselling, u'll have three options in hand -
I- Not satisfied with allotted seat & want to be considered fr 2nd round. Excercising which, u are ready to forego ur seat.
II- Locking the Institute.
III- Freezing the seat allotted, so u won't be considered fr further rounds.
So, if u go with the 1st option.....u may be allotted a higher preference, or u may be given the same seat in the 2nd round depending upon the vacancies.
In any case, u cannot urself choose b/w the branches allotted in the two rounds,i.e. u have to be ready to forego the seat of Round-1 once u choose the 1st option.