This is the USB relay board we recommend for our chassis dyno. This is used if you plan on using computer control for the dyno. (It is not used if you are using mechanical switches).  

Our dyno has 2 styles of loads.  First, the heavy roller acts as a heavy flywheel. Second, the brushless slave motor adds constant repeatable loads.  The Denkovi relay allows the computer to control the 4 load levels instead of flipping mechanical switches. This is ideal so the various load levels are being held exactly the same way every time.  Something that is not really possible if you are flipping switches by hand.