How short circuit is made when connecting negative cable first when jumping start the vehicle
When you have vehicles with the battery negative connected to chassis (mainly the norm), it makes sense to attach the positve jump lead first because, if you accidentally and momentarily touched it to the chassis of the other vehicle nothing bad would happen. However, if you had already connected the negative jump-lead first, then you would short out the battery on one vehicle if you accidentally touched the positive jump-lead against the chassis.


  And could you explain how the short circuit is created?


The battery in any vehicle has one terminal connected to the chassis of the vehicle. If the terminal that connected to chassis was positive (as in older model vehicles) then you would attach the negative jump lead first.