The reactor is powered by a dilithium crystal. Once the flight is underway and when the reactor is heating, stress will occure on the crystal. Balancing the matter and anti-matter streams, the user can protect the crystal from cracking. Best case if that does happen - you end up without power. Worst case - the ship explodes.
The core for this card allows the FD to see the reactor heat, the dilithium stress level, and fluxuate the stress level at random.