It makes your phone more useful and gives you an extra highly customizeable button in the place of Head Phone jack. Its also have a sleave which comes in different colores and can be snaped to headphone wire to avoid misplacing the MiKey when its not in use.

This button can be used for

• Take pictures
• Start Video recording
• Turn On the Tarch
• Start Audio recording
• Scan 2D Code
• Make a call to a slected number
• Send a particular SMS to a selected number
• Open folder of 4 selected apps
• Launch any App from the phone.

System toggle settings

• Screen shot
• Screen Rotation
• Kill all Tasks
• Phone Lock
• Home
• Silent Mode toggle

For example 

• Single Click -> Take a picture
• Double Click -> Open QQ Mobile app
• Tripple Click -> Call Home Number 

and so on upto ten clicks

Oh I forgot to mension that in order to use and configure the MiKey you need to install a free app from xiaomi app store namee MiKey (米建, mijian). This App is in chinese but there is also a translated version of the app (Mikey English) @xiaomi.eu . There is also an other app english app Klick which can be downloaded from playstore. This app aslo work with Mikey but functionality is a little different.

MiKey Functionality when phone is locked/screen is off 

• Even when phone is locked it can still take pictures (older pictures are not visible so no security breach)
• turn ON the torch
• But most of the other function you need to unlock phone by yourself