In what situation do we use “off that”? and what for?
Answer from a Native speaker
The preposition "off" in the phrase "off that" refers to expanding upon an idea that was previously stated. "Off" in this context can be used in any situation where you are having a conversation and you would like to talk more about an idea that someone else mentioned. However, it is a common expression to use in large group discussions such as a class discussion or in work meetings. Ex: Going off that idea of public transportation, I'd like to add that it has become too expensive. Ex: Off that thought, I had a similar experience yesterday.