On column B, 7 is possible in B2 and B4. There is a linked pair such as if B2 is true, then B4 is false or viceversa.
Likewise for C1 and H1 on the first row, if one is true, the other is false.
These linked candidates are connected in block one.
If 7 is true in B4, then it can be excluded from H7. If it is false in B4, it is then true in B2, false in C1, true in H1 and thus false in H7.
In all cases, 7 cannot be in H4.
