Contrast training, from what I understand, actually developed inside the weight room and later moved to other sports. Overspeed for an Olympic lift would simply be lifting, at maximum speed, a weight lighter than your 1RM. Contrast training, also loosely called complex training (there are some differences) involves a very slow, heavy movement, like a near max lift, followed within a minute or two by an overspeed movement, such as a light weight/bodyweight movement. I have read various opinions about which potentiates which. We use max effort squats (1RM to 4RM) followed by a fast movement such as a jump squat, CMJ or depth jump. It probably is best suited for those who have a limited training window. Block periodization would be more effective if longer periods are available.