Well, quite a question.
As of the latest (August 2011) NFL CBA - Incentives have a lot of rules around them. I'm not a lawyer or arbitrator but from what I can tell, Brooks' contract sounds legal (and fairly typical) for a player of his status in the NFL.
As far as what is spelled out around individual player incentives in the NFL CBA - I amSection 6(c).v on page 96 of the CBA states, "Any incentive bonus that depends on team performance in any category not sure this listidentified in Exhibit A hereto is all inclusive or limited to this listprohibited." while section 6(c).vi at the top of page 97 states, but here"Any incentive bonus that depends on a player’s individual performance in any category not identified in Exhibit B hereto is prohibited." so the incentives listed in the CBA are what it says:players & teams are limited to as far as incentives go.
INDIVIDUAL INCENTIVES
RUSHING
- Total yards
- Average yards (100 attempts)
- Touchdowns
PASSING
- Passer rating (224 attempts)
- Completion percentage (224 attempts)
- Interception percent (224 attempts)
- Total yards
- Yards per pass (224 attempts)
- Touchdown passes
RECEIVING
- Total receptions
- Total yards
- Average yards (32 receptions)
- Touchdowns
DEFENSE
- Interceptions
- Interception return yards
- Touchdowns on interception returns
- Opponent fumble recoveries
- Opponent fumble return yards
- Touchdowns on opponent
- fumble returns
- Sacks
PUNT RETURNS
- Total yards
- Average (20 returns)
- Touchdowns
KICKOFF RETURNS
- Total yards
- Average (20 returns)
- Touchdowns
PUNTING
- Gross average (40 punts)
- Net average (40 punts)
- Inside 20-yard line
PLACE KICKING
- Total points
- Field goals
- Field goal percentage (16 attempts)
- Field goal percentage 0-19 yards (4 attempts)
- Field goal percentage 20-29 yards (4 attempts)
- Field goal percentage 30-39 yards (4 attempts)
- Field goal percentage 40-49 yards (4 attempts)
- Field goal percentage 50 yards or longer (3 attempts)
OTHERS
- Roster bonuses
- Reporting bonuses
- Playtime bonuses (excluding special teams)
- Special teams playtime
There are also team incentives where the amount set aside in the pot for each incentive is split evenly among all players on the team. The rules around how team incentive money is calculated into the salary cap is long-winded and written in legalese so I will refer you to the NFL CBA PDF if you wish to read more about it. Do I get some incentive for having back-to-back-to-back 3-letter acronyms in my answer? ;)