[John Valentine]

What’s an “illegal drop”? I’m new to bowling.

[fhayek]

The rules say that in his situation, he was supposed to drop “as near as possible” to his original divot. He admitted that the area was too wet and he dropped two yards back. This establishes that he intended to use the drop to gain a better lie.

[USS Alaska]

What’s an “illegal drop”? I’m new to bowling.

Here is a perfect "illegal drop".