The idea is to tune at full output however, full output places the amplifier under stress in the tuning condition. To remedy that problem, they reduce the duty cycle of the carrier by pulsing it. The biggest problem is those pulses need to reach the same power level as your highest word peaks will and you really need a peak reading watt matter otherwise your just tuning the output stage to match 50 ohms at an average power level at the sacrifice of peak watts and efficiency.
Another thing that has been pointed out is the grid meter (and plate current) will only be showing the much lower average, pulsed grid current during this type of tuning. It's usually noticeably higher during normal operation with a carrier or word peaks. If the grid is too high, you will need to back off on the load and re-dip the plate current back into the resonant null. If that fails to keep grid current within reasonable levels, it's likely the amplifier is being overdriven. Exceeding grid or screen current will get you into trouble faster than exceeding plate current or dissipation (unless your blower sucks).
