In the below sentence:
- the dog that Jake patted t
The constituent "the dog" is said to be extracted from the embedded clause "that Jake patted t", and is coreferent to a gap that is in the object position of the verb "patted". That's all that extraction is -- a relative clause ("the dog that Jake patted") headed by a noun ("the dog") which is coreferent to a gap within the embedded clause (the object of "patted").
Knowing this, what does it mean that "it appears in a location that does not normally permit extraction"?
This means that the inside of the preposition "without" is not usually a position from which extraction is possible. This can be seen as follows:
- Without imagining the book, I couldn't read it.
- *the book that without imagining, I couldn't read t
However, it is licensed by the parasitic gap construction, suggesting that extraction gaps are different to parasitic gaps.
- That's the book that I can't read t without imagining pg.