Prorated signing bonuses: This is the hardest thing to understand about the cap. For cap purposes, signing bonuses are spread throughout the length of the contract. For example, a $4 million signing bonus spread across a four-year contract counts $1 million against the cap each year. This explains why a team can pay a player $5 million in one season but have it count only $2 million against the cap. You add the $1 million base salary with the 2009 prorated part of the signing bonus and end up with a $2 million cap number.
Salary-cap hits: The prorated part of a signing bonus will always count toward that year's cap. But because every dollar counts toward the salary cap, teams must add the remainder of a signing bonus if they cut or trade a player. [emphasis mine] They do this in two ways. The rules are different in 2009 because there is no cap scheduled for 2010 unless there is a collective bargaining extension within the next year. If a player is cut in 2009, the team takes the entire salary-cap hit in 2009. If a player is cut before June 1, the cap hit occurs this year.
Take that $4 million signing bonus example. If that player who accepted a $4 million signing bonus in 2008 is cut this year, the team is on the books for the $1 million proration in 2009 but also must take a $2 million salary-cap hit this season. Thus, the released player would count $3 million under the 2009 cap, even though he's no longer on the team.
Why trades usually don't happen: Often, it's because of that prorated signing bonus hit. Unlike with releasing a player, there is no way that a team that trades a player can delay taking a salary-cap hit. Once a player is traded, the remaining proration is moved into that season. The same standard applies if a player is cut. Trading can begin Feb. 27.