Photographer Ben Von Wong explains:
Motion blurs are created through a combination of movement and long exposures. Essentially, you are capturing colour and light for the duration that your sensor is exposed to light. This gives you three fundamental options: Either you move the light, you move your subjects . . . or you move your camera.
Utilising these choices, you can create some really amazing effects as demonstrated in the video above. For more information on how this works, check out Ben's full post over at DIY Photography.
How To Capture Motion Trails While Freezing Subjects Using Shutter Drag [DIY Photography]