The barrel has a pivoting link underneath it that is pinned to the frame. The link allows the barrel to move with the slide as it moves back under recoil, then the barrel is pulled down by the link and pulls away from the slide to allow the slide to travel to the rear to eject the empty case and load a new round.
The slide and barrel stay locked together for a short distance to allow the bullet to clear the barrel and pressures to drop to safe levels before the breech opens and the brass is ejected.
A short youtube video animation of the 1911 operating cycle:
http://www.youtube.com/watch?v=YKRMcTlbWTs
You can clearly see the barrel move back, then down on recoil and up and forward into battery when chambering the next round. Note the lugs on the barrel above the chamber that lock into grooves in the slide, these are what keep the barrel and slide together during the cycle.
Mark