1) First, buy some good relays (see below). Buy an amp rating that is about twice the load you'll using. For example: if you're using 4-6 amps, buy a 10 amp relay. For this application, I'll use a 10 amp Crouzet relay. Note the model #. Crouzet has recently changed item numbers for this part:

Crouzet G240D10 = Crouzet 84134000 = 10AMP, 5-24DC
Crouzet G240D25 = Crouzet 84134011 = 25AMP, 5-24DC

Another brand of relay I like is Omron:

Crouzet G240D10 = Omron G3NA-210B-DC05-24 = 10AMP, 5-24DC
Crouzet G240D25 = Omron G3NA-225B-DC05-24 = 25AMP, 5-24DC

You can buy relays just about anywhere. I use alliedelec.com or grainger.com.

2) Relay terminals 1&2 are for switching your AC line; terminals 3&4 are for the DC control line. When terminals 3&4 have voltage applied (anywhere between 3 and 32 volts for this relay), the relay will close the "switch" between terminals 1&2. When 3&4 doesn't have voltage, terminals 1&2 are "open".

3) Kill all power to circuit at the breaker box before continuing. Now, tap into the power line for the target 400W light. Snip one leg of the wire, strip back 1/2 inch or so, and connect terminals 1&2 "inline". The relay will now be able to "switch" the power to the 400W light via terminals 1&2.

4) Next, prepare the B2102/B9006 (see images below). In these pics, I've used CAT-5 wire; use that or something with color-coded wire. Let's pretend we're wiring this 400W light to channel 1 of the B2102. When using the B9006, channel 1 on the B2102 maps out to terminals 1&2 on the B9006 (3&4 are channel2, etc). Terminals 1&2 on the B9006 are being "switched" by the B2102 (via software).

5) Connect one CAT-5 wire to terminal 1 (solid blue in jpg); connect +12VDC to terminal 2 of the B9006; connect other CAT-5 wire to -12VDC (white/blue in jpg). Now run your CAT-5 wire to the location of your relay. Connect the solid blue wire to (relay) terminal 3; the white/blue wire to terminal 4.

6) Since the relay needs 3-32 VDC to switch it's terminals 1&2 (the AC leg), we'll use the B2102 to "switch" 12VDC to the relay. When we switch channel 1 of the B2102 "on" (close B9006 terminals 1&2), it will allow 12VDC to flow to the relay (on terminals 3&4). Since the relay now has between 3-32VDC on terms 3&4, it will switch terms 1&2 on ("closed") - the light will now come on. To turn it off, we switch channel 1 again and the 12VDC drops, which will cause the really to "open" terminals 1&2, shutting down the light.