You might use this feature to have a player lean around an obstacle to see something for instance. The value returned is pre-filtered for an optimum combination of stability and latency and is packaged to resemble the thumbstick API.
Leaning left and right corresponds to X movement; leaning forward and back corresponds to Y movement. The values range between -1 and 1 in both directions, where 1 roughly corresponds to 45 degrees of lean.