I don’t like the idea of sliding hands along the rails, I think that’s a bad practice. Granted, holding side rails is how it’s shown in the picture on the OSHA site, but my personal experience with OSHA and my opinion is that a few of their methods (like ladder climbing and nailing cleats for scaffolding to walls) are wrong.
If you grasp the rungs you can catch yourself if you slip, even with one hand. If you’re holding onto a side rail, chances are you won’t, you’ll fall.
To descend safely- I believe more accidents happen while descending- you don’t release your grasp on the upper rung until you have a grasp on the next rung down. With 30 years as a carpenter, I’ve caught myself a number of times this way. Never had a serious fall.
The safest way to climb or descend is to maintain three-point contact… you only lose contact with the ladder with one foot or one hand at a time.