This mod brings in the Extra Elytra option from the Wurst Client.
Must be installed on server for mod to work
If the mod is installed on the server and client, you can accelerate with W and decelerate with S while flying with the elytra.
Also, if the Height Control option is enabled, you can ascend with the spacebar and descend with shift.
The elytra related code uses the Wurst Client code as is.