The process through which branch change happens here at IIT  is as follows:(for 2018 and 2019 batches)

First the grades in courses MA1101 and PH1010 are considered. Then in case of a tie(which almost always happens) the GPA is considered.Thus a priority order is created.

Some important rules are that 1)only net 10% students may add to the branch 2)only 10% students may subtract from a branch.

This means ; say EE has 130 students ,10% of which is 13. But say if 2 EE guys go to CS, then 15 can enter EE.

But the most interesting rule is as follows

Consider a student from Biotech wanting to shift to EP. Let's say he has a GPA of say 8.9 with a A grade and a S grade and is listed in priority order with rank 29. Also EP has not satisfied its 10% branch change quota.

But owing to his bad luck , he is not able to be in the top 10% cgpa holders of the biotech department (who are eligible for branch change) which rules him out of the branch change race.