The on-call alert fires at 2:47am. The service that is degraded was written by a team that no longer exists. The runbook links to a wiki page that has not been updated in two years. You have the codebase, the metrics dashboard, and a Slack thread with three people asking when it will be fixed.
This is the condition under which you need to be able to read a system you did not build. Not in a week, with time to study the code. Now, under pressure, with incomplete information.
The four-step method works under those conditions. It uses the frameworks from the preceding chapters as lenses rather than study materials — you read the system through them rather than reading about them separately.
The method is also useful in less urgent conditions: joining a new company, inheriting a system from a departed engineer, preparing for a system design interview, or onboarding to a codebase for the first time. The urgency changes; the method does not.