Fast looks like you are putting away a pair of six-shooters in a cowboy movie. You start with your hands in a fist, with the index fingers and thumbs extended, like forming the 'L' sign horizontally, as if to shoot a gun from both hands. Then pull both hands back toward your torso with your index fingers curling inward at the same time (almost like forming the 'X' sign in ASL).
Fast is typically taught around the same time as slow, so that you can teach the meaning of each word by drawing the comparison between the two words. You can teach animal signs as well, like a horse or fox being fast, and a snail or turtle being slow.
As your baby begins to discover and explore mobility, you may want to introduce them to contrast signs associated to velocity in movement. You can teach this sign by having play dynamics that include fast and slow movements.
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