On the Slack Engineering Blog, Felix Rieseberg writes:
In practice, switching the analysis and the compiler on without changing code means that TypeScript will immediately attempt to understand your code. It uses built-in types and type definitions available for third party dependencies to analyze the code’s flow, pointing out subtle errors that went previously unnoticed. Wherever TypeScript cannot understand your code, it will assume a special type called “any” and simply move on.
Read more from the source: Several People Are Coding