Let me try!
Both Rixter & Ozperp have explained it well.
You have property A, worth $500,000 and a loan of $300,000 with Bank X. For the purposes of this example, you already have a LOC with your loan to 80%. The loan documents with Bank X state the security for this loan is against Property A.
So you see a new property, lets call that Property B, that you want to buy and you want to take the $100,000 and use it to purchase it. You will purchase it for $400,000. (I am leaving out stamp duty). You go to Bank X and say I want to buy Property B, will you give me a loan. The bank says yes for a loan of $300,000. The loan documents for Property B with Bank X, say security for mortgage is Property B. In this scenario you are not x-coll.
If however, in your loan docs the security details for the mortgage states both Property A & B, then you are x-coll.
Extracting equity to pay for another property in itself has nothing to do with being or not being x-coll. Using the same bank however, leaves it open for it to happen, you need to be explicit with your instructions to the bank.