The principle
Knowing your rival is critical. Use spies to gather valuable information — or better, become the spy yourself by mining ordinary conversation for what is not being said.
The application
The literal version is unethical at best, criminal at worst. The literacy version is: most professional gold is leaked casually by people who would never write it in an email. Learn to listen for what is volunteered when no one thinks they are sharing anything.
The defensive tell — how to spot it being run on you
When an acquaintance from outside your industry suddenly takes a deep interest in your work — asking unusually specific questions framed as fascination — the framing is the disguise. Answer warmly and at the level of public knowledge.