Mac: When you click in the well of a scrollbar in Mac OS X, one of two things occurs, depending on your preference: you either jump to the location you clicked or you scroll up or down an entire page length. In general you'll choose the scrolling option you prefer, but sometimes you'll want to switch things up. Fortunately, all you have to do to reverse your scrolling preference temporarily is hold down the option key.
Check out the video above for a demo.
This shortcut can be hard to understand when described, so take a few seconds and try it out for yourself, using this window as a testing ground. Try clicking in the scrollbar well (the empty space that the bar travels inside of) in the window and see how the bar scrolls. Now do the same thing while holding down the option key. You should notice the behaviour is a little bit different. The OS X default is to scroll one page at a time when you click in the well, but you can also set this to jump to the specific location where you click. Sometimes it's useful to be able to use both and now the option key will let you do that.
Want to do a lot more with the option key on your Mac? Check out this post on its other hidden functions.
Five unexpected uses for the Option key [MacWorld]