The three cells R2C1, R2C9, R3C7 form an Ywing for the candidate 8. When R2C1 is not 8, it is 7, hence R2C9 is 9 and R3C7 is 8. Therefore, the candidate 8 has to be placed in at least one of the yellow cells R2C1 and R3C7 and therefore can be removed form the green cell R3C1.
