Concentric, or bottom up squats are initiated from both a dead stop position and also a more relaxed state. Rapidly going from this stopped and relaxed starting state into a tensed movement is tremendous for developing explosiveness and hence is great as a way to improveate of force development.
When trying to build explosive strength it is best to use a light to medium weight and try to explode up as fast as you can. In order to get the most out of this exercise ensure that your muscles are fairly relaxed before each rep so that the emphasis is on developing high levels of force quickly. To help with this, remove yourself from under the bar for a second or so between reps to give yourself a chance to relax your working muscles.
Some good alternatives to concentric squats are:
Concentric squats are no doubt a good vertical jump exercise but if you want to increase your hops in the fastest way then you need to check out our book Game Changers: The Most Powerful Vertical Jump Training Methods Known To Man.
This book contains special techniques that produce results far above regular jump training drills. Get your copy today.