unconfirmed transactions – Instantly Accept Bitcoin Payments (0 confirmation trx)

GAP600 is a service (that has processed $5 billion in cryptocurrency) which allows exchanges and payment service providers to process Bitcoin payments instantly (0 confirmation trx). It guarantees a transaction against double-spend, expiration or other fraud even before it is confirmed by the Bitcoin network:

GAP600 facilitates instant, risk-mitigated cryptocurrency commerce by
enabling exchanges to recognize unconfirmed Bitcoin.. transactions as
final using a proprietary risk engine, which analyzes and performs
live risk scoring for each transaction as it reaches the mempool.

How is it possible to guarantee a transaction against double-spend, expiration or other fraud before it is confirmed by the Bitcoin network? What kind of “risk analysis” might be performed to achieve this result?