What makes a slow pitch delivery legal?

Under Rule 6C, the pitcher must:

• start with both feet on the ground and one or both touching the pitcher's plate (Men's, Women's and Coed C, D and E/Rec may also start from in front of the plate, with feet within its 24-inch width),
• come to a full stop for at least one second, ball in front of the body and facing the batter,
• deliver underhand on the first forward swing past the hip, in one continuous motion with no stop or reversal, keeping the pivot foot on the plate until the ball is released,
• throw at moderate speed, judged by the umpire (a pitcher who repeats excessive speed after a warning is removed as pitcher),
• give the pitch a perceptible arc, reaching at least 6 feet and no more than 10 feet above the ground,
• release the pitch within 5 seconds of getting the ball, and not deliver it from the glove.

Pitchers get three warm-up pitches in one minute at the start of each half-inning.

Illegal pitch penalty: if the batter swings at it, the illegal pitch is cancelled and all play stands. If the batter doesn't swing, it's a ball.

For batters: a pitch that lands on the ground or home plate can't legally be hit. It's a ball (dead ball) even if you swing at it after it lands.