We
use an algorithm, to determine when a player is eligible to move to a new tier,
which takes into account a number of factors including number of games
played. These numbers scale by tier, so
as it's necessary to shoot a lower average, more games are also needed to reach
higher tiers. For example, when you
reach Tour Master, you will need to play a larger number of ranked rounds than
you had as a Master, maintaining a low average, to tier up to Legend.
Here is a good thread to read:
http://www.wgt.com/forums/p/9085/707139.aspx#70713