Dash instant payments (one of Dash's strongest features) are made possible by transaction locking on the Masternodes. Basically you have to trust that 8 out of 4000 random Masternodes are good actors. Because the choosing of those 8 are out of the senders control there is no easy way to game the system. So the trust lies in percentages in combination theory which makes the system very secure with bad masternode actors being removed immediately.