Skim
Skims are the tech you'll likely use the most when wallriding since it's the tech that gets you around the map the fastest. A skim can be described as a tighter jump. Every input is as short and close to each other as possible.
In practice this means that your first jump input should only be a tick long to reduce your time on the wall to a minimum. After the first input you perform the 2nd input on the next frame to reduce the time between the wall drop-off and wall jump.
Execution
A skim, like any other tech, relies on the same basics as a jump. Attaching and detaching from the wall with the first input and doing the wall jump with the 2nd. Reducing the time spent wallriding to a single tick, however, lets you keep almost all your speed, because the wallriding can't slow you down fully in only a single frame, making it perfect for going fast. Reducing the time between the wall drop-off and wall jump reduces your overall jump length, letting you perform more skims, and gain more speed, over a shorter distance.
Pros and Cons
As we can see, skims are the ideal tech for movement, being the fastest possible tech, still being very easy to do, and therefore also being consistently hit. However, they're not the perfect fit for every situation. Sometimes you require more distance to the next wall than a skim can give you, or you want to turn a sharp corner. In both of these situations skimming becomes a crux. Skimming also won't allow you to get the maximum speed out of jumps started from ground speed, due the single tick of holding the wall not bringing you up to full wallriding speed.
Tips
A small tip for all keyboard and mouse players:
Using scroll wheel jump for skimming will make it a lot easier to hit the tick windows and will make skimming much more consistent. Concerning the speed at which you should scroll, a wise man once said: Not faster than a sane person would pet a cat.