Before each match is played, one player will roll a die to determine which tier they will choose their teams from. Rolling a 6 means you roll again. Once the tier is determined a coin flip decides which person gets to pick two teams from that tier. The other player will then get first choice of the two teams chosen.
Based on previous tourney results the tiers are tweaked a bit every year, adjusting for whether a certain team dominated or struggled or how often a team was selected.
Here are the tiers for Tomczak Bowl VIII