Herculean strength is required to press together the two ends.
Hold the handle together and use the attached clip to keep it closed while you squeeze the other handle closed.
Sometimes if the victim is in a lot of pain, and it is clear that you will not be able to effect a release, put the victim with the trap still on into a container. Go to a vet who can sedate the animal, remove the trap, and treat the injury. Rehabilitators are the best people to call for this. They usually list themselves with the local police or veterinarians.
Caution: In some states, disturbing or removing a trap may expose you to legal actions by the state and trap owner.