According to the <a href="https://en.wikipedia.org/wiki/Helmholtz%27s_theorems" class="extiw" title="en:Helmholtz's theorems">Helmholtz theorems</a>, any span-wise change in lift (and consequently in <a href="https://en.wikipedia.org/wiki/circulation" class="extiw" title="en:circulation">circulation</a>) causes the shedding of a trailing vortex. The strength of that vortex is equal to the difference of the circulations left and right of the considered element.
<i>Part of a series of illustrations meant to illustrate the <a href="https://en.wikipedia.org/wiki/lifting-line_theory" class="extiw" title="en:lifting-line theory">lifting-line theory</a>.</i>
<a href="//commons.wikimedia.org/wiki/User:Ariadacapo" title="User:Ariadacapo">Olivier Cleynen</a>
<span class="int-own-work" lang="en">Own work</span>