Drain chiles, reserving liquid. Coarsely chop chiles; place in a 6-qt. electric pressure cooker. Add tomatoes, onion and reserved drained liquid. Combine garlic, salt, cumin and cayenne; rub over roast. Halve roast; place in pressure cooker. Lock lid; close pressure-release valve. Adjust to pressure-cook on high for 80 minutes. Let pressure release naturally for 10 minutes; quick-release any remaining pressure. A thermometer inserted in beef should read at least 165°.
Remove roast; shred with two forks. Remove vegetables with a slotted spoon; discard cooking juices. Return beef and vegetables to pressure cooker; heat through. Serve in tortillas, with toppings if desired.
Freeze option: Freeze cooled meat mixture and juices in freezer containers. To use, partially thaw in refrigerator overnight. Heat through in a saucepan, stirring occasionally and adding a little water if necessary.