The collective bargaining agreement
The 100-per-cent rule
50.7 "100 Percent Rule" for Multi-Year SPCs. The difference between the stated Player Salary and Bonuses in the first two League Years of an SPC cannot exceed the amount of the lower of the two League Years (or if the same, that "same" amount). Thereafter, in all subsequent League Years of the SPC, the variation in Player Salary and Bonuses from one League Year to another may not exceed that amount.This is the portion of the CBA Tom Benjamin is referring to when it comes to differing values between years of a multiyear contract, which is illustrated with this example (one of eight provided): "An SPC provides for $2 million in Player Salary in Year 1. In Year 2, the SPC may not provide for less than $1 million, or provide for more than $4 million."
What that means is that many of the various loopholes speculated on over at Sabermetric Research would be invalid. It's also fair to assume that any other type of extreme salary cap circumvention would be frowned upon by the league.
UPDATE The most extreme cap circumvention possible under the terms of the CBA appears to look something like so (taken from a commenter on Benjamin's site):
Cap hit: $2.188-million
As far as I can tell, there aren't any provisions in the CBA to prevent a deal like this (although I'm still looking).