LONGDESC

I find myself suddenly confused about the LONGDESC attribute. According to 
http://www.w3.org/WAI/wcag-curric/sam3-0.htm, the LONGDESC attribute 
resides in an image tag such as the following:

<IMG SRC="graph1.gif" LONGDESC="graph1.htm" ALT="3-d sales chart.">

But when is access to graph1.htm presented to the user? Is this IMG 
supposed to be a link to graph1.htm? Is the UA supposed to render a D link 
to the LONGDESC URI immediately following the IMG object? Is an AT supposed 
to simply announce the URI? I assume that the average user isn't supposed 
to view the source, copy the URI. and paste it into the address edit box 
just to find the LONGDESC text, right?

Can someone educate me?

Thanks,

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