The fed govt supports a given number of places at each med school. A few years ago they wanted to increase the number of doctors in shortage areas, so they introduced the bonding scheme. Each med school must have a given number of students (26 at UNSW) in the bonded scheme, so to ensure that they do get that number they will usually bond more people than necessary at the start of 1st year. Once numbers are finalised, they are then free to unbond people to bring it back down to the exact number. Those to be unbonded are chosen by who has the best combined UAI/UMAT/interview scores.
People who don't put down BMPs reduce their chances of getting in at all (because pretty much everyone puts CSP as 1st pref, you need to be of the best 146 to get one); people who do put BMPs as a preference have a greater chance of getting at least a place, which has the slight chance of being unbonded.