I know what you're talking about and it's one of the many cool gestures that the trackpad on your MacBook, MacBook Air, or, if you're using the "magic trackpad" bluetooth trackpad from Apple with your iMac or similar, lets you accomplish if you've enabled it.

And that's the trick: by default most of the multi-finger gestures on your Mac are not enabled by default. On the other hand, the system preference area where you can tweak the settings of your trackpad is one of the most informative and interesting in the entire operating system, with over a dozen training videos built right in.

Let me show you how to enable Screen Zoom and you'll see what I mean about all the videos embedded.

Step one is to choose "System Preferences..." from the Apple menu on the top left corner of your screen. Once you're there, look in the middle of the window for "Trackpad":

Click on "Trackpad" and you're in the zone:

Move your cursor around: as you move over different options for the multi-touch trackpad, you'll see the video changes to demonstrate both the gesture and result in a short video. Brilliant!

The feature you want to enable to get that cool zoom feature is Screen Zoom:

As a tip, click on "Options..." and tweak it just a bit too while you're at it:

I find that when I have Screen Zoom enabled on my MacBook Pro I like to also choose to have the screen image also move so that the cursor is near the center. Otherwise you can get a bit lost when the cursor is off-screen.

I'll also note that screen capture utilities act very weird when you've zoomed your screen, so if that's part of what you want to accomplish (perhaps recording some training videos) you've been warned.

Still, even if you don't plan on enabling any of the multitouch trackpad capabilities, I encourage you to check out how incredibly well Apple's designed this particular system preference, with built-in training and help information. If only the rest of the operating system were that well designed...