Free association is where you use what the girl is saying to create what you are going to say on the fly.
This can be used as a technique to create natural sounding and effective conversation.
Part of how you do this is by changing what you say on a topical level in a way which keeps her engagement level just right.
The "Topical Delta" is the difference in the topical subject matter between what she just said vs. what you say immediately after her.
For example, if she told you a story about 4 topics such as her dad, horseback riding, travel and apples, you want to tell a story about 2 of her topics and 2 of your own new ones such as: apples, travel, diet, and your crazy cousin.
If the topical delta is too big then what you say in response to her will seem too weird.
If the topical delta is too small, then she will think the conversation is boring.
So, in other words you need to follow a few rules:
1. You must always add new information to the conversation (or she will get bored).
2. You must always say something contextual to the conversation you are in by making sure your topic has something, anything to do with what was just said.
3. You must never parrot back what the other person just said, this will create a stall. If someone does this to you, then you must start a new conversation about a different topic.
When combined with changes in the types of conversational elements you are using (comfort, attraction, edge, sexual innuendo etc.) the result is always fascinating conversation.