I stumbled on this little trick as I was preparing selection sets for facial rigging. I am dealing with a geomety if the head that is complex and convoluted, while at same time being a single unified mesh. If you have seen screen shots of the geometry know what I am talking about. The head geometry is isolated from rest of the body, and is merged with body geometry downstream from facial rigging SOPs. Still I am dealing with a lot of vertices, and becouse the geometry has many folds, and overlaps, creating selection sets is timeconsuming. You can't just drag a lasso around a region. To create a selection set I first isolate it in view by hiding all points that won't be in the set. Still its a PIA, but it takes less time. So accelerate the selection for hiding I use this little trick.
Select Edit SOP but do not RMB, the selection will be used with Visibility. Single click on vertex (DO NOT RMB).
Switch to Select Primitives (faces). At this point Houdini will select all faces that share the vertex. Do not RMB
Now switch back to Select Points. At which point Houdini will select all points that the highlighted faces share. You'll notice that point selection set has grown.
Repeat this proccess to grow the selection as needed.
In my case, when selection begins to approach vertices or faces I do not want selected, I hide the currently selected set, and begin again. This proccess continues untill only the vertices/faces that I need for the Edit or Transform SOP are left on screen. I simply select all of them and RMB to create the SOP. Then unhide all primitives in 3D. At that time, I can do whatever I need to the selected set without fear of grabing extra points.

Dave Rindner