You are more or less “shaking” them, or maybe “firmly jiggling” would be a better description. It’s not like the shaking you would use for a cocktail, but neither the word “swinging” nor the word “rocking” really describes the motion you want either. If the baby is really crying hard, you don’t want your motion to be too “gentle”; you want to match the intensity of their crying with your motion and shushing.

Try to find a video demonstrating; it’s quite hard to convey the appropriate amount of vigor with text.