What is webhook?
A webhook is a URL you register so the platform can call you rather than the other way round. Twin sends two kinds: a job completion callback carrying the result, and a monitor change callback carrying the previous hash, the new hash and an excerpt. Delivery is best-effort with retries and never blocks the work that triggered it.
Why it matters
Polling a job you expect to take ten minutes wastes both sides. The cost of a push model is that you now own an endpoint that must authenticate what arrives — which is what the signature is for.