To answer your question, the same as if you disconnect the negative. However the positive being re-connected is more dangerous than the negative mostly because when your tigtening down the positive your tool can touch a metal surface causing a bad arc, if you look at your factory manual under roadside emergencies you will notice that they say to connect the positive first and the ground last.
http://www.motorcraftservice.com/pub.../12focog4e.pdf
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Troy.
