It is were you use a sp. tool called a breach  setter to  push a bullet ahead of the chamber into the rifling about 1/16" to 1/8"! This measurement not set in stone!

     Breech seating is the use of a cartridge case usually on the end of a tool to mechanically seat the bullet into the rifling of the barrel. By using this method the bullet is usually seated into the barrel the same for every shot.
     The bullet can be seated so that when the loaded case is inserted behind the bullet the bullet can be anywhere from slightly inside the case neck to any distance in front of the case.  It is usually seated anywhere from 1/32 to 1/4 inch in front of the case.
     The primed cartridge with powder. and sometimes a wad to keep the powder from falling out of the case. is then inserted behind the bullet and the rifle is then fired.

The only stupid newbie questions are the one's that are never asked.My father owned a Sharps-Borchardt, it shot wonderfuly well with breech-seated bullets, it would not shoot beans with fixed ammo.Nuff said.

broken_arrow;
                    Welcome, hope you say with us
              All serious Bench "stool" shooters breechseat their bullets--fixed ammo will not shoot with breechseated bullets--you push this soft lead bullet up ahead of the case and it is lined up with the bore and can do nothing but go straight down the barrel---    leadball

Thanks for the replies.
Now I understand.

How about a couple of follow-up questions...

Is this done only with blackpowder or is it done with smokeless also?

If the bullet is placed ahead of the case mouth, do you get flame-cutting of the bore in the area between the two?

Thanks again,
dave

Breech-seating probably developed in the formal target shooting games of the German/Swiss Americans as a result of their schuetzen muzzleloading heritage.  Remember that most of this developed before clean-burning smokeless powders became available.  Pushing a bullet down the muzzle "pre-fit" the bullet to the bore and the patch at least partially wiped the bore clean of the fouling from the previous shots.

Early in the schuetzen game when breech-loaders came into play they would still load the bullet from the muzzle and insert a primed and powder filled cartridge case from the breech.  Elaborate and very precision made false muzzles were made for the better target grade rifles for especially this purpose. ( Don't ask my how choke bores and gain twists worked with them---maybe they were a later)  It was part of the tradition and helped with the bore fouling issues.

As smokeless powders came into play and BP residue became less of an issue it was found that muzzle-breech loading, as it was called, was no longer required.   However the pretty much hand made--or at least hand finished--barrels still required very careful matching of bullets to bores and chambers.  The relatively soft bullets could easily be damaged by loading into the case as well as the problems mentioned with neck tension, the bullet to bore jump etc etc.  Using a mechanical seating device to carefully place the bullet into precisely the right position in the barrel was a fairly logical step and it has over the past 50 years or so years become pretty much the hallmark of revived Schuetzen rifle shooting.
However not everyone does it, and even some those who do it most of the time use fixed ammunition on some occasions.  
Some modern shooters keep a small supply of very carefully preloaded fixed ammunition on hand to take fast advantage of specific wind conditions even if it is not quite as precise as a breech-loaded bullet.  
Some offhand shooters use fixed ammo.  I'd guess that their theory is that their natural body movement creates more angle of dispersion than they would gain by adding the extra steps of breech-seating, so why waste the time and effort.

The schuetzen riflemen of the golden age of the sport shot a lot of different kinds of matches in the course of an event and looking at the surviving historical records its kind of hard to tell exactly when they used breech-seating and when they did not.