What's the payload of a subscription?

Looking at http://developers.littlebitscloud.cc/?javascript#create-subscription

What’s included in the payload of amplitude, amplitude:x:x, etc.? It seems the subscription is working fine and keeps invoking the http endpoint, but there is no data. Is that the goal? Just an invocation? I was looking, e.g., to record the change in temperature from a sensor, etc.

Thanks!