Introductory it as Subject
Using it and the verb be to focus
When you want to focus on an element of the sentence, you can put it and a form of the verb be at the beginning, followed by the element you want to focus on, then a pronoun such as who, which or that, then the rest of the sentence. Thus instead of saying John got married last week, you can focus on John and say It was John who got married last week. The meaning of this is often contrastive: It was John, not Paul, who got married last week. This structure is often known as a cleft sentence.
Her height is striking enough but it is her face which amazes everyone. (Her face amazes everyone.)
He was at Hove yesterday, and it was a fair bet that it was Alan Wells who he had gone to watch. (He had gone to watch Alan Wells.)