Freezing motion on apature setting using a flash

I have a 30D with a 580EX flash. I prefer using the apature setting to gather light and show motion. This usually requires a slow shutter speed which is good for motion but the subject is sometimes too blurry. It appears the flash fires at the beginning of the exposire. I want it to fire at the end to freeze the action at the end of the exposure. Does anyone know how to set the camera/flash to achieve this?

It's called second-curtain sync and you can set it with a custom function on your camera (on a 20D it's Cfn-15 set to 1).

RTMiller wrote in post #2176880
It's called second-curtain sync and you can set it with a custom function on your camera (on a 20D it's Cfn-15 set to 1).

But just be aware that 2nd curtain is only available with ETTL compatible flashes attached directly (or via an extender) to the hot shoe mount. So it's not going to do any good to set the custom function for 2nd curtain and then use a non-ETTL flash or trigger a flash, ETTL compatible or not, via the PC sync terminal.