THE LMR says that if you are an eligible employee on the 1st day of the last month of your tax year (usually 12/1) you are eligible to contribute for the entire year. However, if you use the LMR, then you must remain eligible through 12/31 of the following year. this is the testing period referred to.
see this article which provides more details
https://livelyme.com/blog/last-month-rule/
