The best I could tell was that OPs voltage inserted isolates the TV-end HDMI power pin and only sends power back toward the Pi. According to OP this causes issues with some smart TVs so he adds the diode as a jumped to add power to the TV-end pin too. The diode prevents power going back toward the USB end but I can't imagine a scenario where this would happen.
The cheaper voltage inserter I linked above doesn't isolate the TV-end power pin (so power is sent both ways down the HDMI cable) and therefore doesn't need to be modified (unless there is some reason for the diode over a jumper that I'm not aware of - in which case the explanation from those above is inadequate due to the way the pad works, you'd need to detach the red wire and put the diode between the wire and the pad).