A wheel under the roller shoe.
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/56783767@N00">Dan Taylor</a>
CC BY 2.0
<a href="//commons.wikimedia.org/wiki/Flickr" class="mw-redirect" title="Flickr">Flickr</a>: <a rel="nofollow" class="external text" href="https://www.flickr.com/photos/56783767@N00/392836616">Heelys</a>