[Ronbo]

Blu427 is right, with the neg disconnect it eliminates possible shorting of the "still hot" positive lead going to the switch and the vehicle body. Simalarly, when changing a battery it's important to disconnect the negative terminal first to eliminate a wrench slip shorting the positive terminal to the body.

Current is consistant across the circuit so the "spark" is the same regardless.

With either side disconnected the vehicle can still run, although the electrical system has now lost it's main filter (the battery) so electronic ignitions can malfunction and cause the motor to either shut down or run rough. Also some alternators will shut down with the battery disconnected.