The Cambridge rules are a random transfer STV method. The Cambridge rules also use the stack model, but with a variation. Instead of taking the ballots on the top, the pile is "sliced." This means that every Nth ballot is selected starting from the top of the pile. N is chosen to go through as much of the pile as possible.
Secondary surpluses are not allowed. When transferring votes, once a candidate has reached the winning threshold, she cannot receive any more ballots. Any ballots that would go to this candidate instead go to the next choice on the ballot.
After all surplus votes have been transferred, all candidates with fewer than 50 votes are eliminated. This is related to Cambridge's requirement that one must collect 50 signatures to be put on the ballot. It is also a matter of convenience as any candidate with fewer than 50 votes at this stage has virtually no chance of winning a seat.