I am not 100% sure, but i think backordering works like this: say i own appletreeinthegarden DAWT com, and you want to buy it, if i didnt select the protection or something during sign up you can backorder it, meaning that when the registered time of the domain comes to an end i will get a email telling me Hey!if you dont renew your registration within 1 month we will sell the domain name, then if i'm a active webmaster and use the site i will just go and renew my hosting and all is well for me, if i dont renew it in the specified time they sell the domain to you.
Not sure just guessing here