I was only thinking in terms of internal and equivalent resistance/impedance at the connector and more generically to a circuit with parallel power sources and one load.

Your explanation of the internal trigger circuit makes sense. When I measured voltage with my old VOM (with relatively low internal resistance), the voltage measured somewhere less than 2 volts. When I did the same with an O-scope with 10x probe (I think it's 10MOhm internal resistance?), it measured 5 volts - indicating that impedance is pretty high.

Do you have an actual diagram? If a small cap is being charged with high impedance source at the primary side, how would it build up such a high energy enough to generate a flash? Somehow, I'd think the trigger circuit is separate from charging circuit.

Geez, it's cold here... my fingers don't move smoothly! (and I'm in Florida!)