Slow Sync Flash is an interesting technique you can use to create photos in dark conditions. It takes quite a bit of experimenting (and a bit of luck) to get it right. You’ll get the best results in a low light setting but with some colorful lights in the background that you can use to paint, and then when the flash goes off you freeze your subject.
If you want to dig a little deeper into slow sync flash, I recommend this article.
Here is how you set it up on your Canon 550D.
Select Menu > Flash control > Built-in flash func. settings > Shutter sync. > 2nd curtain.
Now for the fun part:
Exit menu, and try setting your shutter to 0″3 or slower and experiment.