For a better comprehension, gave a small modification to the view above, added the wave-form-layers to see the current going further;
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Click image for larger version. 

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We can only melt steel sheet with the 'Welding Current' of the pulse, while we make repose when the pulse is off.
This repose-period when the pulse is off, we call the current of this period 'Base Current'.

Thinner sheet are easy to be melt-down, so we beginner love to use this trick, melt the target in short period, then make repose to cool the target down for a while, and repeat and repeat.