No, you connect your dummy load to the RF output on the Thruline. The inserts are purely for the scale of power you use.
So if you have the 100mW slug inserted it means that you use the bottom scale of the meter and the maximum read out would be 100mW at the top of the scale on the meter. (same with 1w, 10w, 100w and so forth)
If you're using the 50kW slug you'd use the middle scale of the meter up to 50, same goes if you're using 5w/50w/500w etc.
The usual power meters like Avair and Daiwa for example use the same method but you just select the power range via a switch and then multiply the read out accordingly.
