Total newbie question: 8 ohm vs 4 ohm

Over the years, I have heard a lot of discussion about amp
power at different impedance levels. Could someone give me
a basic clue explaining in what situations this is
important? My speakers have always been 8 ohm, so I haven't
had to worry about this too much. However, I have seen
reviews of certain speakers, like Infinity Kappa 9's, where
the owners say that the impedance drops as low as 2 ohms
when there is a lot of sound going on. Also, what effect
does this have when bi-amping or bridging?

To someone with out an EE, this is a little bit confusing.

The ohm rating on speakers is not constant, depending on the frequency the resistance may increase or decrease. So, an 8ohm speaker will average out to be about 8 ohms, while it may drop below 6.5 ohms at times during frequencys that the speaker can handle very efficiently.

the ohm level is the resistance the speaker has to the current going into it. Most amps will handle a 4 and 8 ohm load, but on some amplifiers a 4 ohm load just works the hell out of it. Some amplifiers can handle resistance loads all the way down to 1 ohm.

think of it like this, if you are blowing air through a straw you can do it for a while before your lungs empty out. if you blow through a paper towel tube which is obviously alot bigger and allows more air to flow through (less resistance) your lungs will give out alot faster.

the only problem i could see with bi-amping is if you are using a speaker that fluctuates resistance drastically like a ESL speaker, if you have an amp that can handle it driving the lows while an amp that cannot handle it driving the highs, yer likley to blow yer tweeter. switch around vica verca.

you really should have to worry about it alot, go by the speakers specs, you can usually trust that an amplifier capable of 4 or 8 ohm loads can handle either, but it an amp can only run 8 ohms and you hook up a 4 ohm speaker, you might collapse one off yer amp's lungs.