It is much like a traditional DI box, that takes a feed from the guitar (pre or post pedal chain) before the amplifier input (pre - amp)...
Except for it takes a feed from the amplifier's speaker output (post - amp) and a speaker cable in the pass-through connects to the speaker cabinet.
So that an un-effected signal can continue playing out of the amp while the direct box taps a feed after the effects+amplifier to capture the tone you are looking for. The XLR output is brought down to a safe "mic level" just like a traditional DI to be plugged in to a sound board's mic input, an audio interface, or an outboard mic preamp.
Combine with a mic'd cab and blend to taste for interesting results or use it on it's own.