Threshold is a tool common to PSP and PS in which you set a tonal level and everything brighter is turned to white and all below are made black.

This shot was soft and rather than throw it away I played with Threshold to make the rather stark image on the right. Some liked it, others didn't, so I then added a shot taken earlier that morning, First the right way up, although mirrored for better composition, and a third layer flipped for the reflection in the water.

 There are always different ways of doing things and threshold can be used to make a mask. With the flowers the black stamen as easily removed with the rubout tool and the mask inverted and coloured with a pattern.

In PSP Threshold is one of the options in Adjustment Layers and was used to create the stark area around the girl. Using the paint-out brush to remove the layer over the normal shot of her. The advantage of using ALs is that if you make a mistake you can paint the layer back in again.