First I suggest leave the stick at home for now and concentrate on the carrot - i.e. what is the perceived benefit for the firm/your current team? I'm assuming you are performing well on your current team, well liked etc and they would be reluctant to see you go, hence the query. Stress to your current team that it is important to you as an individual (appealing to their empathy): it is an opportunity you have been interested in since you joined and would not want to miss out on getting that additional experience and bandwidth. It's the same firm, even same dept, so you'll have line of sight to one another. Underline that you want to keep warm ties with your current team. Unless they want you gone you cannot really build an argument that losing a resource (you) is in the interests of your current team (at least not in the short to medium term) but you CAN appeal to their sense of fairplay - would they not want to take the opportunity if they were in your shoes? By all means say it is for the short term - chances are they will replace you anyway, and if they are busy they wil just get on with it.Underline at the company level that gaining this additional experience in the other team makes you a more valuable and flexible resource, that you can make an impact on the new team, and are highly motivated to get more exposure INTERNALLY rather than looking outside. Show them a positive, cheerful demeanour and someone looking to milk their potential for all its worth and most people will back you; start stamping your feet in a tantrum and threatening to leave to get what you want and you might just find yourself either between 2 teams and part of neither or part of a much larger team which meets at your local jobcentre.
