The gesture progress at which this trigger is active. The gesture has a progress from 0..1 measured across it's length.

<Panel Width="100" Height="100" Background="#000">
    <SwipeGesture ux:Name="swipe" Direction="Up" Length="50"/>
    <WhileSwipeActive Source="swipe" Threshold="0.5">
        <Scale Factor="1.5" Duration="0.4" />
    </WhileSwipeActive>
</Panel>

The Scale will apply as soon as the user swipes 25 points, 0.5 of the total Length.