I have not heard this saying, but it makes sense to me in this way: a trader can enter a trade by placing a limit buy order below the current price, this will be filled when selling is going on. This makes sense because you will incur the maker fee, which is less than the taker fee (incurred when you place a market order).
Exit via the sell side: if you are long on a position, do not market close the order, set a sell order to close your position for the size of your current position that you want to close. Again in this case you will not incur the high taker fee, but the lower maker fee, thus increasing your profits (as long as it was a profitable move, of course).
I hope this helps
its my first post here.
