[quote name='Mr. Foxen' timestamp='1340463399' post='1704809']
In practice they mean a constantly changing impedance load on your pickups, which changes the sound coming from them. Buffered bypass is superior for consistent sound and to avoid the switching noise: http://www.tech21nyc.com/technotes/index.html
[/quote]
Link above is from a company which makes pedals. Not really unbiased stuff.
Whether true bypass is something you want is another discussion; it is a real thing and not simply a large switch or marketing bollocks.
Yes, it changes your pickups' impedance which, in turn, changes the sound coming from them. That is also why people buy pedals!