What is meant by the IRS "testing period"?

If an accountholder contributes based on the Last Month Rule, the IRS requires an individual to remain eligible for all of the following calendar year or pay income taxes and a 10 percent penalty on any amount contributed in the current year that is in excess of the maximum amount an individual could have contributed using the Sum of Monthly Contribution Limits Method. The idea of a "testing period" is to prevent an HSA accountholder from getting more of a tax deduction for HSA contributions than they are entitled. The IRS testing period is a requirement where an individual, enrolled in an HDHP, has to remain on the HDHP until December 31 of the following year to not be "over contributed" in their HSA.