How it (hidden motors) works article here... I found it interesting...
http://cyclingtips.com/2015/04/hidden-motors-for-road-bikes-exist-heres-how-they-work/
It appears that the rider will be spinning the motor and the bevel gear set at all times?
Unlikely to be a positive gain in a long ride?
And likely, a permanent magnet motor, the eddy current drag would be an issue.