It stands for balanced transformerless connection. Looking at the manual, it looks as if that switch sets the two amps into bridging mode.
This is risky to a degree, as if you do it wrong you will do a lot of damage. If you use this amp for the center in bridged mode, then you need to make sure you connect to the correct input terminal and the speaker to the correct output terminal.
Now there is one big caveat to all of this. When an amp is bridged, it tolerates only twice the previous impedance. So you need to make sure your center channel really is an 8 ohm speaker. The truth is that is is very unlikely your speaker is an eight ohm speaker. Manufacturers lie about this all the time. So you need to look for independent measurements about this, or measure it with a Dayton woofer tester yourself. If your center is a four ohm speaker, and it likely is in the power band, then your amp in BTL mode will see a 2 ohm load. That could easily spell disaster.
Since the difference between using just one of the amps in the Kenwood rather than two bridged is only 3db, your safest approach is to use just one of the amps in the Kenwood and not two. Many amps have gone to the recycling center after being bridged.