You contact someone (usually via the rent trade boards) who has points they are looking to transfer. After receiving whatever fee you both agree on they call MS and ask for the points to be transferred to your account. Once in your account they become your points and you have total control over them, the person you got the transfer from can't "steal" them back.
Word of warning: Although transferred points are supposed to retain the use year and home resort of the person who you got them from, in reality this is rarely the case. i.e., I had some VWL pts with a july use year transferred to my account. They took on my use year (december) and home resort (BWV) when they were transferred. From what I have read on these boards that is what happens pretty much all the time. If you DON'T want that to happen to you, be sure to tell MS and request a manual transfer of the points to retain the use year and home resort of the person you got them from.
HTH!