Electrical connection during car jump start
The negative terminal of a car battery is connected to the car's chassis and engine block, so connecting the negative jumper cable to the chassis does (eventually) make an electrical connection to the negative terminal of the battery.

The lead-acid batteries used in cars can generate hydrogen and oxygen gas while being charged.  If you make a spark in the presence of these gases they may explode.  It is recommended that you make the last connection of jumper cables some distance from the battery to reduce the danger of explosion as making that last connection will very often make a spark.