A Diminuendo Binding, as the name suggests, is a binding cast by a djinn where the subject of the binding shrinks and turns them extremely small. It is often used in djinn duels where a diminuendo binding is placed on one of the pills for each person.
In The Blue Djinn of Babylon, Izaak Balayaga first mentions this binding when he meets John and Philippa on the Royal Hungarian Express train from Istanbul, Turkey, to Berlin, Germany. He says that they were lucky that he didn't use a diminuendo binding on them, which would have turned them into dolls until he said otherwise. It is a binding that is usually placed in something that the subject of the binding consumes, such as a chocolate-covered ant or a tasty English sausage.
Later John accidentally casts one on Groanin which was meant for Zaregus. Due to it having being hidden in the sausage, John had no choice except to turn him back aftwerwards. However, due to Zaregus's influence he couldn't remember his focus word and Philippa had to go inside his head and find it out for him.
Only the djinn who cast it can reverse a diminuendo bindings as which explains why they are used in djinn duels, and reflects the permanency of use when it was cast by Iblis or even John.