Here's a photo that illustrates how the bag is set at an angle to prevent heel strike. The top rail with the hooks is at angle, as is the rail on which the bottom catch is secured. All three are fully adjustable, so the top hooks can be placed either side of horizontal rack struts and the bottom catch can be adjusted to fit snuggly against a down strut to minimise movement. Properly fitted, it doesn't move at all.

If you look at the largest size, you can just make out the bottom catch if you follow the line of the tubus rack.

http://www.flickr.com/photo_zoom.gne?id=252416656&size=o