overtraining is a very long process and builds up over time, one doesnt become overtrained overnight, most people believe they overtrain if they do to many sets or whatever, but they are overstretching , not overtraining. theres a difference, when you lifts start to stagnate or your cns doesnt fully recover between workouts that overreaching, overreach for to long and you get overtrained or get overtraining syndrome...

look up 'dual factor theory'...