Earning the spread?

I wasn't 100% sure which forum to post this in. If I've picked the wrong one I apologize. But I've seen people (or websites) talk about having the ability to "earn the spread" using ECN brokers. Could someone explain to me what is meant by that? Is this a trading strategy of some kind? And what brokers allow for it?

Similarly, what does it mean to trade inside the spread? Is this the same thing?

On an ECN you can place buy/sell limits inside and at the current best bid-/ask-quotes. You can try to buy at the bid and sell at the offer (or inbetween, thus narrowing the spread). Naturally, it is not sure that your orders will be filled.

...or only one leg/position is filled and than the market moves in the wrong direction to your filled pos. Than no spread is earned, only a loss.