Cross Matching how does it work?

I've noticed in some match bets that if a team is 1.5 if you put up 3.25 on the 2nd team as a bet. The 1.5 will automatically turn into a 2/1 on the other team and get rightly ahead of you.....But sometimes it doesn't.

Does anyone know how it works. If I put up 9/4 on the 2nd team when 1/5 is available on the other team in a match.

Will when the other team is blank except for my 9/4. Will somebody who doesn't know better automatically end up on the 1/5 on the other team. If he's tempted to match my 3.25.

Making my offer of 9/4 on the other team a waste of time if its a quiet market.

Does anybody know how it works. Will the 1.5 be automatically matched instead of my 9/4?......Thanks.