The 1, 2...1, 2, 3 rule is my own way to keep the speed constant. The first movement, whether you are going down or bringing up, is to a speed of 2 counts. The eccentric movement, the second movement of the machine or exercise, is to a speed of 3 counts. This is to ensure that you are not doing it too fast or too slow.
*Too fast can actually increase the risk of pulling or tearing a muscle.