[exim] SRS Help

I could use some help getting SRS to work with Exim. I've got the
libsrs_alt library, and I've re-make'ed Exim with SRS support turned on (I
can see it linked in libsrs_alt.so). It's what the router section needs
to look like that's got me. From the example I have:

srs_router:
driver = redirect
srs = reverseandforward
data = ${srs_recipient}

as my first router, but it doesn't seem to be doing anything. Can someone
better explain:

srs = forward
srs = reverse
srs= reverseandforward

are these conditions? When are they true?

Thanks!