As you know, I make it a point to trade "The Gap" every Friday. While I missed my specific trade last weekend, I got caught with two open orders. The both gaped in the correct directions giving me $400+ profit.
I liken the Forex to a river. It's level is constantly changing throughout the week from people getting in and out. Over the 48 hour weekend, it settles or moves into equilibrium. Thus, a gap is formed.
It's just an easy way to look at it.