In the best cases, it's totally automatic. You know that it worked at commit A and is broken by commit Z. So it checks out commit M and runs the tests. If they succeed, then it broke somewhere between M and Z. If they fail, then it broke somewhere between A and M. So it checks out either H or S, depending, and repeat.

It's not always that easy, especially when your tests and environment are complicated. There's often manual intervention, which is tedious. Still, log2 N steps is often manageable, especially if the computer is taking care of the tracking for you.

The other day I used it to write a good bug report. I first used it to find the earliest commit I could compile on my machine, then I used it to find the commit where a certain command would fail.