Understanding Exhausts and Timing

Hello,

I understand how timing works, but I am trying to wrap my brain around how changing an exhaust affects timing. What is it about putting on a performance exhaust that makes a change in timing necessary. And how do you determine if you have achieved the timing you need for a particular exhaust? It seems to me that advancing/retarding the timing would make a timing light obsolete? I can only speculate. I really don't know.

Is there a simple explanation for this?