Essentially it's a split bet. Here are two scenerio's
You bet on Spain -0.25 +110 vs Italy and wager 200. You have the following. 100 on Spain at PK +110 and 100 on Spain at -0.5 +110
You bet on Spain +0.25 +110 vs Italy and wager 200. You have the following. 100 on Spain at PK +110 and 100 on Spain at +0.5 +110
Hope that helps.