As Paint Shop Pro becomes more powerful so do its tools. This power comes with change, though. Certain tools, such as masks, must, necessarily, change their behavior.
Changes, such as these, can be a little disconcerting to users especially with tools as complex as masks.
In version 5.xx, masks have changed quite radically. Part of this change is due to the fact that PSP 5.xx users now have the capability of using layers (for more info on layers, see the tutorial "PSP 5-Layers Primer").
Masks in PSP 5 are now more like masks in the higher-end programs such as Adobe's Photoshop and Corel's PhotoPaint.
The basics are still similar in that a mask is grayscale and where the mask is white you won't see any change and where it's black you will see change.
The change you will see, though, comes not from applying effects to the image through the mask, but from what's on the underlying layer(s).