The only caveat is that the public financing mechanism is strictly optional, as the Supreme Court has ruled that the right to donate money, however much the limit is, is a 1st Amendment protected right.
In Arizona and Maine, you usually have to collect X number of 5 dollar donations from registered voters in your particular district equal to X percentage of voters in that district in order to demonstrate viability. Usually, we're talking about a 5 percent rule or something. If you can't meet that threshold, you get no funding.
Once you get funding, you get a lump sum payment, but you must abide by the rules so you don't get it yanked. Such rules are you voluntarily agree not to accept any donations from individuals or groups, agree to abide by spending guidelines, and agree not to use your own money in the campaign like some wealthy candidates have done. With Arizona and Maine, if your opponent outspends you, you get matching funds within reason. (If the guy spends 100,000,000 to beat you in a district in Arizona, the state government won't give you 100,000,000 to defend yourself without breaking the bank)
There's a video at the top that explains that and other rules regarding such a system:
http://caclean.org/materials/