Steve,

Electrical things can be weird. The battery acts like a big sponge so when a charger is attached it senses this load...plus when you hit the starter on the vehicle being jumped, the draw is quite dramatic...several hundred amps at least.

Maybe this is the load the portable jump needs to see before it does something internal or electronic, I don't know. What kind of load does a winch motor see when you kick it in?

Sometimes the fellas at the parts store do know who's on first as they are right where the action happens. Did you 'Google' this question...lots of info out in cyberland.

Stuart