Miband Band Extension Plate - to register activity when cycling and using similar gym equipment

The Xiaomi Miband is a popular low cost health band (i.e. wearable technology). Users may want this simple band extension foruse when cycling or at the gym using an exercycle (or similar).
Like most health bands the Miband features a pedometer (plus a range of other features including monitoring sleep etc.) and is usually attached around your wrist and functions through movement. So how does it track steps?
When you walk or run your arms swing and these devices register that movement... n