This trick used to be seen a lot more often than it is now. We have a nasty feeling that it may have resulted in a few accidents, and performers have decided that the risks outweigh the rewards when it comes to doing it successfully.
There's really no trick to this at all - the sword really does get swallowed. The performer very carefully pushes the blunted blade straight down their esophagus, which they've spent years training to accept the foreign object. One wrong move and they'd be in real trouble.