How to do this iPhone trick?

I was reading an article detailing some less-known iPhone tricks. One tip was this:

Feel and see when people call: Apple has added a number of accessibility features to iOS 5, specifically designed to assist those with hearing, vision, mobility and other disabilities. For example, those who are hearing impaired might opt to have the LED flash when a call comes in. If you're seeing impaired, you could set a unique vibration pattern for different people in your Contacts, so you know who's calling.

But it doesn't explain HOW to do it. I've searched settings but I can't seem to figure it out… Does anyone know? First person to help me get it wins 10 points.

Setting, general, accessibility, led flash for alerts.

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