nope wouldnt worry about.
occasionally when a turbocharged car hits peak torque after a sharp rise you can see a C like that, and it could be the same here. the dyno load overcomes the engine for a split second and forced the rpm to drop. it could be the same here, but the drop in torque would seem to indicate he let off the gas for a second right as the dyno loaded up. pure speculation, ive never even seen a dyno. :ninja