Boris from Meldium here. One of our customers wrote a great description on our blog (http://blog.meldium.com/home/2014/5/19/the-web-setup-envy-la...) which I'll paste here:

"The actual implementation continually blows my mind. You're basically using a man-in-the-middle attack for good, if I don't betray my poor understanding of the concept. A less malignant analogy would be to liken it to the way telephone companies operated switchboards before individual telephone numbers connected people directly. You had to call the central and ask the operator (i.e. Meldium) to talk to your friend Frank McFakeperson in Ipsum, South Dakota (i.e. MailChimp) and the operator would patch you through to them seamlessly."