Hi jimhsu,
I'll try to help. There are lots of different types of matrices and the term itself refers to an interlinked geometric structure/design. Some can create spillover and some don't just like some pay commissions for referrals and others pay nothing unless the entire matrix structure below someone is full meaning the person at the top "cycles". The numbers used to describe them usually refer to width first then depth. Here are some examples:
A 3 x 3 matrix usually means that once the first person gets three people under them their first level is full so if they get a fourth referral that person gets placed under the first person on their first level. Their matrix would be full once they had 3 people on level one, each of their level one refs had 3 refs making 9 people on their second level, then each of those people having 3 referrals putting 27 people on the first persons third level. In that case it would take a total of 39 people to "fill" that matrix.
If it were 3 x 4 then another level would be added so the matrix wouldn't be full until all the people on level three had 3 people each under them or another (27x3=) 81 people were added.
Some matrices aren't "forced"--the term forced referring to the limited width of levels forcing spillover to the next level.
Basically it is best to read through the explanation of any matrix you are considering joining, run the math and make sure it makes sense to you.
I hope this was some help and that I didn't confuse things more.