How to add solder traces on PCB surface
They removed the solder mask and the board was wave soldered.
Dave Jones found it reduces the resistance of 1oz copper by 50%, so the equivalent of 2oz copper in resistance (and higher in thermal capacity, which might come into play for surge currents). His methods were good. He depended on looking up the copper thickness.
Mike of mikeselectricstuff found a 30% reduction. His methods were a bit less good than Dave's but good enough (verified when Dave removed the solder and it was verified that the thin remaining layer did not significantly affect the resistance). I did not see him attempt to find the thickness of the copper- he assumed it was 1oz. If it was actually 2oz then the two empirical measurements would agree, close enough.
eg.
1 || 1 = 0.5, a 50% reduction 

0.5 || 1 = 0.333, a 33% reduction 

If the PCB maker went a bit light on the copper thickness then the relative improvement would be better.
It has two advantages- it's basically free (especially compared to thicker copper), and it's selective (only the traces that need it can be so treated).
It might be good to find a more rigorous or official source. A quick look on my part does not yield one.