If my guess is correct (swarming doesn't seem to be
documented) it tries to download chunks of a file from
multiple sources (if available).
This does not seem to work very well for me. I suspect it
may be related to my using a modem. I know from Gnucleus
that such multiple source downloading is NOT recommended
for modem users. Yet GTK-Gnutella's tooltip seems to
strongly recommend using swarming.
Any advice for a modem user? Should I keep swarming on or
should I turn it off?
Most of the files only ever have 1 source anyways.

Upload speed in 56K modem is about 1K or 2K so swarming should help the problem comes when you selfish idiots tie up all the downloads available with this stupid selfish idea then you all lose hahaha on you

There is no reason to turn it off. Leave it on.

And don't forget to share your files