These links describe very clearly how it works (in summary – it’s better NOT to feed at the low impedance point like a dipole as when you double the frequency the same point is now a VERY high impedance – it’s better to accept a little mismatch and get lots of bands instead.

For more detailed explanations (with pictures :-) ) see:

* Good OCF Introduction
http://www.radioelectronicschool.net/files/downloads/ocfdipole.pdf

* Clear explanation but shows the difference using a 20% feedpoint can make -it includes 10MHz)
http://www.w8ji.com/windom_off_center_fed.htm

* A more extensive article with various shortened versions
http://www.stroobandt.com/cl-ocfd/windom/index.html

or without the frame:
http://www.stroobandt.com/cl-ocfd/windom/windom.html

OCF Antennas are sold commercially from
  http://hamcall.net/7bandocf.html

and the user reviews on EHam.net do seem very positive:
  http://www.eham.net/reviews/detail/5838

These are still full length antennas (e.g. 40m/130ft if you want 80M without using an ATU) but without the anti-resonance that makes a 80M dipole hard to feed on 40M (for example), and having the feedpoint at 20% or 33% can sometimes be more convenient.

Cheers
Giovanni – ZL2BOI

I tried the G5RV (in the days before I had an antenna analyser) and didn’t like it much – can’t quite remember why but (from memory) it wasn’t that easy to tune. This corresponds to what I’ve found on the net – the G5RV was never intended as an all-band antenna, however by making it just a bit shorter you can match it easily on lots of bands.

It was derived by Brian Austin ZS6BKW (now G0GSF) and will match most of the HF band excluding 10 & 21MHz.

Have a look at these two articles from Sprat (the journal of the GQRP Club – http://www.gqrp.com):

Martin G3UKV trying it out (with Antenna Analyser results):
ZS6BKW Antenna Revisited (from Sprat #129)

A more extensive article by its designer G0GSF/ZS6BKW, on the rational behind the antenna design:
ZS6BKW revisits his G5RV variant and describes its evolution

Sometime I’ll publish my measurements, but for now, these should give you something to work with …
Giovanni – ZL2BOI