At which progress should this trigger become active.

The default is 1, meaning the trigger will only become active when the page is fully reaches the matching state; partial page progress will be ignored.

Using the threshold we can lower the point at which the trigger is activated.

<Page ux:Class="MyPage" Color="#FAA">
    <WhileActive Threshold="0.5">
        <Change this.Color="#AFA"/>
    </WhileActive>
</Page>
<PageControl Active="B">
    <MyPage ux:Name="A"/>
    <MyPage ux:Name="B"/>
    <MyPage ux:Name="C"/>
</PageControl>

As the user swipes from B to C the progress of B will reduce from 1 towards 0 and the progress of C increases from 0 towards 1. The Threadhold="0.5" here causes the trigger swtich at the mid-way point of the transition. In this setup this means that one page is green and the rest are red -- the one closest to active is green.