The cells E5, F5, F7, H7, H9, B9, B8, E8 form a Unique Loop with the values 9 and 5. There are exactly two ways of placing the values 9 and 5 in the cells of the Unique Loop, forming two possible configurations.
In both configurations, each row, column or box touched by the Unique Loop contains each of the two values 9 and 5 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 Loop can be valid. Note that the two cells F5 and F7 are the only possible positions of the value 9 in the col F, thus, one of these two cells will contain the value 9. Therefore, the only possible way to avoid the two configurations is if neither F5 nor F7 contains the value 5.