re-read my last post cos i've edited it. however, the electric starter motor draws a huge amount of power when it starts. possibly 600 amps or so (a headlight bulb draws about 5amps by comparison). there is an earth (negative) path from the battery negative terminal, usually to the body/chassis of the car, and then on to the engine block. these are very thick, short lengths of cable. if any of the connections are corroded, rusty, loose, or worn, then their ability to supply a return path for 600amps is much reduced. effectively this means that the poor connection is probably able to carry enough electricity for all the normal functions, but when you switch on the starter it can't cope. thus, a jumper cable could 'bridge over' the poor connection and make things work. it's a simple, quick test. i hope i've explained it better this time!!