Timing questions

Okay, true newbie questions here. I just think the answers might come in handy on race day.

Timing a road race is easy. Even if the mile splits are included, it's fairly straightforward. Click start at, well, the start. Click laps at each mile (or whatever interval). Click stop at the end.

For a triathlon, I realize that the transitions add a new dimension to timing. So here's the question. When to I click the lap button to make the change from swim to transition and so on? I've run through several scenarios in my head, and looked on line.

So, I turn to all my triathlon mentors for assistance. What are the appropriate locations for changing from race timing to transition timing and back? Or do I just look for the timing mats and go by those?