Concentric training is basically when you start an exercise from the bottom instead of the top. For example, you begin your squat from a dead-stop bottom position instead of starting by lowering the bar from the standing position. This is typically done in a power rack off pins. (Remember, "concentric" just means the lifting portion of the movement. "Eccentric" usually means the lowering portion or negative.)

This can be done with the front squat, back squat, Zercher squat, or overhead squat. You can start from different heights to break through your individual sticking points.

Related:  The Top-Half Squat

Related:  How to Improve Squat Mobility & Stability