In practical terms it means you are going to have major problems if you use an amp designed for 8 ohm loads with a 4 ohm speaker.
I had a relatively low powered amp (30 wpc class A, dual mono) which sounded real good. I decided to upgrade my speakers and bought a pair of Focus .7s. I was constantly kicking out the protection curcuitry in the amp. I brought the amp into the shop and had it checked out. They said there was nothing wrong with it but as soon as I began listening it did it again. Next I brought in the amp, pre-amp, and speakers along with their respective cables. The guy checked each piece out and didn't find anything wrong. I don't think he ever set everything up and listened to the system as a whole. After years of this I ran into a friend in the HiFi business in Milwaukee and explained it to him. He immediately said "it sounds like you're driving a four ohm speaker with an eight ohm amp."
I bought an amp capable of driving four ohms and never had another problem with the speakers.
I had a relatively low powered amp (30 wpc class A, dual mono) which sounded real good. I decided to upgrade my speakers and bought a pair of Focus .7s. I was constantly kicking out the protection curcuitry in the amp. I brought the amp into the shop and had it checked out. They said there was nothing wrong with it but as soon as I began listening it did it again. Next I brought in the amp, pre-amp, and speakers along with their respective cables. The guy checked each piece out and didn't find anything wrong. I don't think he ever set everything up and listened to the system as a whole. After years of this I ran into a friend in the HiFi business in Milwaukee and explained it to him. He immediately said "it sounds like you're driving a four ohm speaker with an eight ohm amp."
I bought an amp capable of driving four ohms and never had another problem with the speakers.