As the Amendment Explanation says, this restructures the Senate terms so that half the chamber is up every two months, in a similar manner to the RL Nebraska unicameral legislature, which elects half its members every two years.
Simply put, it makes little sense to give At-Large senators shorter terms than Regional and Subregional ones, given that every senator has equal power. This amendment simplifies the calendar by merging the timing of the Regional elections with the Subregional elections to be held in the middle of the President's term, while At-Large Senators are elected with the President.
Originally, I read this amendment as removing midterm elections entirely, but after I realized that this confusion came from a misread, I may be open to voting in favor. (I was originally vehemently opposed to this amendment.) I have more thoughts that I'll edit in later -- a bit busy tonight -- but wanted to put this statement down for posterity.