To spread the lips choose positive, and negative to move them closer together. This is demonstrated in the first example, to get the look of gleeful surprise. Positive values drop the corners of the mouth. Negative values curves the brow up. Positive values move the inner brow up and the outer brow down.
Negative values have the opposite effect.
To pucker move the dial positive and to stretch the lips out move to the negative. The lower lips tucks up under the upper teeth with a positive setting, and with negative values, the lower lip juts out. The pout below is a perfect example of mouth F with a negative value. Set to 1, the lips are positioned to be to produce the M sound. To press the lips together use a positive setting and negative to spread out the lips.