When Jumping start, I understand that I need to connect the positive terminal first due to the safety reason. And also if I connect negative terminal first and touching the body of the vehicle with the wrench by accidentally, I might damage the battery due to making a short circuit. And could you explain how the short circuit is created?
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.