Hooking up a subwoofer, worried about impedance.

My family recently found a set of 5 surround sound speakers, so I got them a receiver and hooked everything up for them. Now, they've acquired a subwoofer from a friend, but I am a little hesitant to hook it up. It does not use the single-channel subwoofer cable that I am used to. Instead, it has a Left and Right input, as well as a Left and Right output. Also, it is not amplified (no power cord). I was able to shine a flashlight inside and see that it was designed to be hooked up in parallel with your existing speakers (the positive terminals of the inputs connect directly to the positive terminals of the outputs, and same with the negative terminals). Several questions arose...

1.) Why the heck would a subwoofer need left and right inputs? How does this even work? There only appears to be one speaker inside, so what happens when you try to hook this up in parallel with your left and right speakers at the same time?

2.) Assuming it's possible, can I safely power the subwoofer in parallel with the left and right front speakers? The front speakers are 6 ohm, and the subwoofer is 8 ohm, so I'm looking at 3.4 ohms. I bought a Yamaha receiver, and I've read that they can handle low impedance. The specs for it can be found at this link: http://www.yamaha.com/yec/products [...] =ATTRIBUTE. It's a little unclear how low you can go with the impedance. The 'features' page says this:

While the 'specifications' page implies that it can go all the way down to 2 ohms:

3.) If I try to hook the subwoofer up in series to be safe, will the volume of the front speakers drop terribly?

Thanks in advance for reading!