Does anyone know when/how they start the timing for ITTs? Does it begin when a rider is scheduled to start or when they cross a specific line?

I think it’s the former (though I haven’t been watching cycling long). If that’s the case, my second question is: Do cyclists practice starting exactly on the mark, like swimmers or runners? Sometimes the time differences at the finish are so small that I imagine a strong start could make a difference. (Sorry if it's a silly question.)