I've always heard that it works like this:
First, poison ivy causes a rash because the oils on it contain
a skin irritant. Your skin has its own oils that form a chemical barrier
from such things. So when you are touching poison ivy, your skin
doesn't really make contact with the irritation for awhile, until
the the Ivy's oil gets thru. You can't see it, so unless you are
careful, you might be rubbing the irritant from one place to another
on your body. But if you wash the contact area within an hour or so,
with lots of soap and water, you can remove most of it without
it getting a chance to cause your skin to break into a rash.
The reason for the claims about Touch-Me-Not/Jewelweed is that
it has more water than most plants and it yields that water easily
with simple crushing. The plant is usually found in the same area
as poison ivy so its readily available.
So it works because its acts like a nearby soapy water faucet, not
because its an "antidote". If the skin already has a reaction, its
too late for any of these things as the damage has been done. The best
that can be hoped for is that it removes any further damage.
jhorvath at bull.com