core/observing

How to be told when something happens in a running world, instead of running something, waiting, and hoping the timing was right.

Covers the probe toolbox: attaching an instrument to code that is already running, the three things you can watch (a function call, a line, a value), what a probe reports and why the text is required, and the case that costs the most time — the world reaching a state you did not put it in, where every value you read means something other than you assume.

Read it when the question is "when did this happen" or "what was true at that instant". For "why does this look wrong" the guide is core/troubleshooting.