Synecdoche is a figure of speech which writers often use in order to show how a part of something can be represented as a whole, and vice versa. So, here, in the sentence Although I was very angry, I did not cause him any harm at all, in order to create a synecdoche, here is what you need to do: Although I was very angry, I did not touch a hair on his head. You can see that the phrase touch a hair on his head is used to denote not do any harm. Just one small part (a hair) is used to represent a whole (harming someone).
