A trait that determines what item a mob will drop will be influenced by its quality, for example, on the yield aspect. In other words: The higher the quality of the trait the more items you'll get from it.
Qualities are: Lowest, Low, Normal, High and Highest.
Place a trait container that has a quality in the slot and provide energy to the machine.power by using rf.
The machine will cycle through the qualities.
If it reaches the Highest quality and then runs once more, the quality will be decreased to Lowest.
The machine will, therefore, run continously until the container is removed from it.
Comparators emit redstone signals correspondent to the quality of the trait inside the machine. The higher the quality the strongest the signal will be.
You can also use BuildCraft gates to measure the quality of the trait inside the machine.