The idea is the O-ring would take up the excessive “head clearance”, that is the space between the case head surface of a fully chambered cartridge and the face of the bolt head when it is forced back on the locking lugs.  This distance by definition should not be very large, perhaps not much more than 0.010.  My No. 4 rifles will not lock the bolt on a 0.070 gage, so most cartridges will have a head clearance of approx 0.004 to 0.006 depending on rim thickness.  

Yes, the shoulder will get blown forward to fill the chamber, but this is not where the problem occurs on rifles with excessive head space. It is the case web that stretches from the head getting blown back onto the bolt face.  The relatively thin body expands radially to grip the chamber walls.  The case body in unable to expand just forward of the head, so that area is free to stretch.  

Don’t full length resize if you want to reload brass more than a few times.  You want to get the case to headspace on the shoulder, thereby keeping the head of the case firmly on the bolt head.  That will minimize or eliminate further stretching of the web which cases case head separation.  

The ones at Wal Mart for hairdressers are perfect, cheap & disposable! They're in the "cosmetics" section with all the hairsprays & so on. A big packet has about 100 or so black rubber bands about 1/2" in diameter. I believe they're called "hairdressers bands".

Its supposed to be done with a light "forming" load, not a full house load & the case is supposed to be lubricated!

(Some say oil others say grease)

Here's how its (supposed to) work.

The rubber forces the head rear into contact with the bolt face, removing any "slop", called "head freespace", caused by differing rim thickness of ammo brands. Basically a tolerance between headspace & rim manufacture.. When fired the lube allows the case body to expand out & forward filling any "generous" chamber dimensions. It slides & slips rather than grabbing the chamber walls & subjecting the case to differential expansion. (which is supposed to cause the infamous case separation woes of the ,303 Brit.)! This is also why thrust on the locking lugs is increased when doing it. Oh and don't forget to thoroughly degrease the chamber & bore after you're done.

Co incidentally it also forms a new neck/shoulder junction matching the individual chamber closely. This effectively makes a kind of pseudo rimless case which headspaces on the shoulder rather than the case rim.

I've never actually done it as I find a technique called "Partial full length resizing" does the same thing faster & better.

i loose track of how many reloads i get out of my cases

with my martini enfield i tighten up the head space by knocking out the dove tail on the bolt face and file fit a new one great for case life