Danger signals. The twisting of the  tail is generally followed by an immediate charge, but in open country like this, one can easily jump aside.
Herbert Lang
<cite style="font-style:normal">Lang, Herbert (1920). "<a rel="nofollow" class="external text" href="https://archive.org/stream/cbarchive_106819_bullzoolsocny19201901/bullzoolsocny19201901">The White Rhinoceros of the Belgian Congo</a>". <i>Zoological Society Bulletin</i> <b>23</b>.</cite>