If true then the busy status is blocked from reaching parent nodes.

<WhileBusy ux:Name="W1">...</WhileBusy>
<Panel>
    <WhileBusy ux:Name="W2" IsHandled="true">...</WhileBusy>
    <Image Url="http://some.place/file.png"/>

In this arrangement the W1 trigger will not be active while the Image is loading. The IsHandled="true" in the W2 trigger indicates it completely handles the busy status for this node and its descendants.