Lol! Obviously everyone knows about the Shift key. What I mean is, not everyone uses it in conjunction with their Brush Tool, particularly while masking.
The principle is perfectly simple - try it yourself and see. Take your brush tool, and click once to paint a dot somewhere on your image. Then hold down the Shift key and click somewhere else. The Brush Tool will automatically draw a straight line between one point and the other.
Now, I guess you mightn't have to paint straight lines all that often, but gradual curved lines are a fairly frequent matter, at least for me. For example, if I'm masking around someone's body, the Shift key is my best friend. Rather than trying to paint the mask freehand, I can do it much faster and easier with a series of Shift-clicks around the subject.
There were only three or four clicks in the above line, whilst holding down the Shift key. In this way, I can mask around an entire person in lightning-fast time. Once I've done the outline, I can easily fill in the rest of the mask with the Paint Bucket tool.