Its easy. You have to be one of the topper.
The better branch you are in, the lesser chance you have. It actually works based on the following rule.

"Strength of a class shouldn't increase by 5% of actual number of seats."

For example, consider a class of 80 students CS. Every other fellow wants to get into it. There is lot of competition for those four branch changer seats. Remember two of them are only available for general category. So, you have to be in top two of the all branches put together except CS. If there are only 78 seats filled during admission, you have total six seats to occupy. But a miracle is necessary for such a fortune.
 
Consider another example of Meta with 120 seats. None of CS, EC, EE, ME, CE, CH wants to enter that. So you just need to be topper from GT, GPT, IMSC, BT, PP, PS etc. Lesser competition as well as some of them might have shifted to CH, or above. And some of the people say in ME  might have upgraded to CS or EE etc. resulting in more number of available seats.