The cells A6, A5, H5, H6 form a Unique Rectangle with the values 4 and 1. There are exactly two ways of placing the values 4 and 1 in the cells of the Unique Rectangle, forming two possible configurations.
In both configurations, each row, column or box touched by the Unique Rectangle contains each of the two values 4 and 1 exactly once. As a result, if one of these two configurations were part of the solution, it could then be replaced by the other one to get a second valid solution.
Because a valid sudoku cannot have more than one solution, none of the two configurations of the Unique Rectangle can be valid. Note that the two cells A5 and H5 are the only possible positions of the value 4 in the row 5, thus, one of these two cells will contain the value 4. Therefore, the only possible way to avoid the two configurations is if neither A5 nor H5 contains the value 1.