Traditional birria uses a tough cut of beef with lots of connective tissues. It can take hours of braising for them to break down, at which point the beef becomes tender. A pressure cooker is a terrific tool for speeding up this process, cooking the meat under pressure often a fraction of the time. If you release the steam and find the beef cheeks are still too tough to shred easily with a fork, seal the pressure cooker again and set it for another 15 minutes to let it cook a little longer.