How did alfred hitchcock create some of the first examples of special effects without modern technology?

Aldrich Hitchcock was considered one of the best English director. He has a technique in getting people's attention through the visual effects of his films where some of it are optical effects and others are mechanical effects.The sound and visual techniques require less attention like his film entitled "Blackmail".
Aside from being a director, he is also a producer and he popularized McGuffin technique.