GentooDepBrowser use an internal representation of the whole portage tree, with both forward and backward (or reverse) dependencies. It's a shame portage still doesn't store and provide this information. Then this is just a matter of browsing this with a friendly user interface.
This internal representation is quite slow to create. GentooDepBrowser creates and uses a cache on the disk, which is updated when needed, based on the timestamp file stored in portage metadata dir. (which is updated each time an 'emerge --sync' succeeded).