From the "What Will They Dream Up Next" department of the Stupid Tax Laws Office:

The 1979 Oregon Legislative Assembly passed the "Two percent kicker" law as a way to control state spending. The law requires the state to compare the revenue forecast for each two-year biennium to the amount actually received and return any surplus to taxpayers.

Voters approved a ballot measure adding the kicker to the Oregon Constitution in 2000 and provided taxpayers with the option to donate their kicker to the State School Fund to help fund K-12 schools in Oregon. After the biennium ends, if actual revenue collected exceeds the forecast amount by more than 2 percent, the revenue surplus is refunded to taxpayers as a tax credit claimed on their personal income tax returns in the even-numbered year of the next biennium.

https://apps.oregon.gov/oregon-newsroom/OR/DOR/Posts/Post/Fact-Sheet-Oregons-Surplus-Revenue-Kicker-...