The BBC also has a system of internal attachments, where staff can apply to do another job for a short period of time (not longer than two years, usually a few months). It's great for staff to get experience in order to get better equipped for full time promotion, and useful for departments to cover maternity leave etc. Quite a lot of regional staff start on attachment to network before making the move permanently.
So that's what Ben Bland meant when he tendered his "resignation". I didn't understand why he would return to Look East.
Yes - in theory the maximum duration of an 'attachment' is two years, and in theory you should return for a decent period of time after this before applying for another attachment.
If there is no substantive job for you (i.e. a permanent position) at the end of your attachment (or if you decide you don't want to stay on), then you return to your previous job, which should have been held open for you (unless that post closed whilst you were away). The people covering the post you left are likely themselves to be on attachment (in some cases someone from the same centre whos is more junior 'acting-up', or in others a similarly qualified person from another BBC centre who fancies a change of scene) or on short-term contracts.
It can create quite a chain - but equally is a very good way of giving people some variety, or some experience in a different job that will benefit them on their return.
However if you've been on attachment in a more senior role, returning to your junior role after a lengthy period of time can be quite a difficult prospect, hence some people chose to leave rather than return.
Attachments are offered for a number of reasons including covering other people who have gone on attachments themselves, covering people who have taken a sabbatical, covering maternity leave or long-term sickness, allowing staff to work on special projects away from their normal job (such as planning special events etc.), covering 'dual working' when a region is re-locating etc.