To feather: how it was done
 

Hi Kath,
It´s just the combination of the slow shutterspeed (1/25 sec)
togehter with the flash.The flash gives a sharp image while the long shutterspeed creates a motion blur,-so you got it both in one picture.(and if you synchronize the flash on the second shutter courtain you get the motion blur behind the cyclists,-I wish I had done this here)
Thanks for stopping by and greetings,
Hermann