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I think that the guy in the video talks to how to calibrate the firmware for the new hotend. It's more than just calibrating the absolute temperature. It's calibrating for the thermal mass and temperature over-/under-shoot so the head settles in on temperature quickly/smoothly and in a controlled manner. He refers to it as "PID" (proportional, integration, derivative) tuning. You can learn more about it here and other tools other than Simplify3D which the guy in the video is using: