There's not too much to add for details, other than:
The gap to jump should not exceed 1/2" for older coils and bikes.
Coils meant to run on higher compression motors 10:1 or greater, should be able to safely go about 3/4".
The coil being tested there is a 1977 KZ650 coil. I was putting it under more stress than it should have on it (it's a throw-away coil, but obviously still works pretty good, at least while dry).
The HEI module is the original 4-pin type. It should be on a heatsink plate, but the one I have there is probably overkill. It should only get slightly warm over 20 minutes or so.
Any standard motorcycle or scooter battery should provide more than enough power to run it for a long time.
The schematic is for a design requiring the coils are 2 ohm or higher resistance. A lower resistance coil should use a ballast resistor in series to make up the difference.
That particular unit is going to my buddy who does a lot of work on bikes.
Getting shocked is all part of the process on a project like that