How does mock-debugger control which line the debugger steps next?

I'm unable to understand how the mock-debugger extension controls where the next step is.

For example what if I'd like to step 2 lines if I find the word "banana" in my text? Also, I'd like to do something, like "Step In", where I can walk word-by-word - is it possible?

I've seen the this._currentLine = ln; assign, which looks like it controls where the line is, but it's just a simple local variable. How could it ever control anything in the debugger? I can't find any other uses of the _currentLine varbiable where it passes to anything useful API (except for stack tracing, but I don't think it has any relation with the debugger line-control).