Dwell Clicking is where you can click by simply hovering yoru pointer over a button or link for a set length of time. This can be essential for head tracker or mouse users that can not otherwise activate a mouse click equivalent.

The following was from a recent e-mail from Natural Point:

The Natural Point dwell clicker does not presently support locking of the left click up/down.
Though there are some free dwell clickers that do have just a left down
click, and a left up:

"Point-N-Click is a FREE stand-alone on-screen virtual mouse designed for
anyone with a disability that makes it difficult or impossible to click a
physical computer mouse." - http://polital.com/pnc/

Sensory Software in the UK also distributes a free dwell clicker on their
site: http://www.sensorysoftware.com/dwellclicker.html

http://www.activeclick.com/

The last free GPL licensed version of mousetool at
http://www.csupomona.edu/~henson/www/dist/mtool31.zip

MouseClick 
www.geocities.com/urifrid/

I like Point N Click the best, though would give all of them a try to see
which one you like best.

Best Wishes,

Kevin Fox
NaturalPoint Inc.

www.naturalpoint.com