What is a forced matrix for MLM?

It's legitimate inasmuch as the program itself is legitimate. What happens is that you are allotted a certain number of referral spaces at the first level, a certain number at the second level and so on. There are two basic ways in which the scheme may work. When someone joins as your referral, they are added to your first level, if spaces are available, otherwise they are added to your second or lower levels as spaces exist. In some systems, your matrix is filled in rotation with others by any new member, as new members join. Whether it is effective or not usually depends on how good the program or product is, how long it has been available and, therefore, how easy it is to get effective referrals to join. Randomly filled matrices are likely to much less effective because you have little or no control over who joins the matrix under you. The most effective programs are those in which the product is worthwhile and desirable and where you have a good upline who supports you well and, as a result, you can support your downline well.

It mean, when the matrix was full they will give their referral to the next level. For example matrix with 3x3, means when You get 3 direct referral.The next referral will go to the next level or 2nd level and so on. And when Your matrix have full, You will have a recycle from the site.