Presumably you are across a current = up and across, across, or down and across. Because you are casting across a current lane, the direction you mend does two things. It speeds the fly as Rip Tide noted BUT it also SINKS or LIFTS the fly in the drift lane.
An easy way to remember what the direction and types of mends will do is to think of dry fly mending. A dry fly mend that would create a drag free drift will slow the fly down and allow it to sink. A mend that increases drag will lift the fly and speed it up.
Most of the time you will be casting over faster water into slower water. For example you will be in the stream and casting into the slower water near the band. Hence an upstream mend as Rip Tide noted across a faster seam will allow the fly to be lower in the water and drift slower. Mend down across the faster seam and the faster seam will catch the fly line and lift the fly.
Unweighted wet flies have a more natural action that weighted flies. So it takes experience and mending skills to get the flies to the level you want. The deeper you want the flies to sink, the further upstream you will need to cast and the more line you will need to mend into the drift lane to get the flies to sink drag free
Remember that you are fishing in 3 dimensions with a wet fly just like with a nymph. But with nymphs you can add weight to the leader but with wet flies you use mending to sink the flies. When mending can't get the flies down, then use weighted flies.
If for some reason, you are casting over a slower seam into a faster seam, just the opposite occurs. If you mend line up onto the slower seam, the line to the fly in the faster seam gets tighter and will lift the fly. So imagine that the wet fly is a dry fly and how you would mend to increase of decrease drag on the fly.
There is a strategy you can employ when fishing a "gang" of wet flies during a caddis hatch. They have this down to an art in England. They do not tie the flies in line but on droppers.
Davy Wotton is a master of this method. Using a very long long rod, at the end of the swing, Davy raises the long rod tip which brings the flies up. Davy then raises the rod so the top fly hangs from the dropper and daps the surface. This dapping will entice a strike for a trout during a caddis hatch.
Fly Rod and Reel Magazine 2012 May - Practical and Useful