The concentric part of a strength exercise is the part where the weight is accelerating, usually pushing up or pulling up or curling up. The eccentric portion of the movement is the portion where the muscle yields to the weight and, ideally, the weight is released in a controlled manner. The muscle is stretched under tension.
In a squat, this is the part of the movement where you squat. The eccentric portion of the movement produces the most soreness and growth stimulus, and is often performed far too quickly.
Eccentric control is the basis of increasing strength. Because you can't accelerate what you can't control. "Release more slowly" and variations of this phrase is the most common coaching cue from a trainer in the YPSI.
A very good variation for advanced users and a plateau buster for exercises where you haven't been able to make any progress for a long time are deliberately slow eccentric movements. Like Medvedev squats.
Former Soviet weightlifting coach Medvedev prescribed extremely slow eccentric movements lasting 8-10 seconds to his weightlifters for exercises like deadlifts in order to develop maximum strength. In the Medvedev Squat, the eccentric portion is 10 seconds. This method is suitable for someone who squats 1.5 times their bodyweight or more and is particularly deficit in eccentric control of the squat.
A LH squats, 10 x 1, 10-0-1-0, 180s
That's 10 sets of one rep barbell squats at a 10-0-1-0 tempo with 180 seconds rest between sets.
Increase to a heavy set over the course of the 10 sets. Repeat every 3-5 days for a total of 6 workouts.
Here is a video clip of YPSI Coach Goran Sirovina doing a Medvedev squat with 180kg
Picture: YPSI Coach Girovina in the bottom position of the LH squat during a YPSI seminar.