You can transfer out to other members as many times as you want during a use year. You can transfer in from other members as many times as you want in a use year. However, you cannot both transfer out and transfer in during the same use year. Transaction simply requires the transferor to call and request transfer to another -- you need the other's member number for that and thus it is often done by a three way call. You can transfer only current use year points not those that have been banked or borrowed, but if you transfer current points the recipient can then bank them. Points are supposed to retain their resort identity when a transfer is made, i.e., if you receive BWV points they remain BWV points for the purpose of the 11 month window. MS has been known to screw that up in the past and make the points received the same as the recipients home resort; that was generally caused by both employee confusion and a computer system that apparently wasn't very good. I heard they have fixed or are fixing that problem.
Note, that transfer is not necessary to allow others to use your points, like when you rent; you can always use your points to make a reservation and put it in someone else's name and that is not considered a transfer subject to the rules above about transfering only in or out or not using borrowed or banked points.