Question: how online checker works?

[Hydoi]

Good question.
From what I understand, no, the transaction will be processed as pending and card holder will not see it. However, merches may be using the same servers to process the transaction, in this case you may end up with having multiple cards been checked from the same non-diversified source. Banks see that and react by banning the cards. To a very certain extent, it depends a lot on your luck.
Never heard of POS checkers