A custom periodic effect with an attack and fade is documented (eg http://qt.gitorious.org/qt-mobility/qt-mobility/blobs/master/doc/src/feedback.qdoc#line162) to behave with a periodic ramping. The behaviour which is observed on device is different to what is currently documented.

In particular:
On N900, no attack or fade is discernible if the effect is a periodic effect.
On N8, the attack and fade appear to apply only to the beginning and end of the entire effect.

Either the backends should implement the documented behaviour, or separate documentation for each platform/device needs to be created.