When powered OFF, UE will send Detach request. Below image shows example of Detach - UE initiated
Data and signalling Bearers will be released on core side and AS side
Once switched ON, it will follow normal attach procedure by sending Attach request
Detailed message flow and contents can be found here for detach procedure
http://www.netmanias.com/en/post/techdocs/6108/detach-emm-lte/emm-procedure-2-detach