In general terms, bi-amping uses two amps per speaker cabinet: one to power the mids and tweeters, the other to power the woofers. Because bass requires a lot more power than mids and highs, separating the two ranges can be very beneficial, especially if you like to listen to loud or dynamic music.
To bi-amp, you first need to remove the metal strap that connects both pairs of terminals on the back of the speaker. This will electrially separate the mids/tweeters from the woofers. Then you run one pair of cables from one amp to the upper pair of terminals, and a second cable from the second amp to the lower pair of terminals.
If you want to take it a step further (as mentioned in the
link Riffman posted), you can install high pass and low pass filters in front of each amp so that only the mids and highs are being passed to the upper terminals and only the bass is being passed to the lower terminals.
