Parameters other than power and duration change the state of the lights by the amount specified.
power and duration act like they do for the Set State endpoint.

For example, to make all lights 10% brighter, and turn those that are off on over 5 seconds, send:

// POST /v1/lights/all/state/delta
  "power": "on",
  "brightness": 0.1,
  "duration": 5  
Click Try It! to start a request and see the response here!