How am I able to jump my car when I ground one of the jumpers?
The battery's negative terminal is connected directly to the car's bodywork.  The whole of the car becomes the equivalent of the negative terminal.

Connect the positive terminals together as per normal, then connect the negative of the good car to somewhere stable on the body of the dead car - somewhere where there is no paint - like the engine block.

The car body will link that through to the negative terminal of the battery, and if there is a spark it will be well away from the battery.

Another way is to take 2 sets of jump leads.  Connect one set to one battery, and the other to the other battery, then link the jump leads together between the cars.  Be careful not to let the + and - leads touch in the middle though.