What happens when Satan is released to test the millennial generations?

Satan is released to test the new generations born during the thousand-year kingdom who never personally made a choice. This proves the innate rebelliousness of the Adamic nature, and when Satan comes back for that specific purpose, those tested generations again follow Satan.

Les explains that after Christ returns and sets up His glorious kingdom, He rules for a thousand years of perfect rule—no poverty, no war, no depressions, nothing amiss. The resurrected of all the ages rule and reign with Him, and a whole new generation of human beings repopulates the planet. All those new children born to the believers who came in at the front end have never had to make a choice. That is why Satan has to be released. When Satan comes back at the end of the tribulation, it is for that specific purpose: to try and test these new generations who have never before made a choice. And again they are going to follow Satan.

Who are the new generations that Satan will test after the thousand years?

They are the children born during the thousand-year reign to the believers who entered it at the beginning. These new generations never had to make a choice, so Satan is released at the end to test them and prove the innate rebelliousness of the Adamic nature.