A batter-runner may OVERRUN first base and return directly to it without liability to be put out. 

If she attempts to ADVANCE toward second base and decides to RETURN (to first) she is at risk to be tagged out.

However once the PITCHER has control of the live ball WITHIN THE PITCHING CIRCLE the following items may also apply:

•  The BATTER-RUNNER may ROUND first base (towards 2nd base), STOP momentarily, then immediately WITHOUT STOPPING again …

•  RETURN to first base.

• OR she may ADVANCE to second base.

 Provided the BATTER-RUNNER stops ONCE momentarily then either ADVANCES or RETURNS, it shall be legal … but is at liability to be tagged out if the pitcher makes a play on her.

This includes a BATTER-RUNNER advancing from a walk, dropped 3rd strike, or any legal method while the ball is live.

For more information on rule differences between high school, amateur, and independent softball organizations ... CLICK HERE.

Rules Reference
NFHS 8-7-2; 8-6-8; 8-7-4 / ASA 8-7T(3); RS #37 / USSSA 8.2; 8.10
NSA 8-8i / PONY 9-8t  / BABE 7.13; NR

If the batter PREVENTS the ball from ENTERING the strike zone by any method OTHER than hitting the ball OR a pitched ball HITS a BATTER while the BALL is IN the strike zone:

The play is considered a DEAD-BALL strike.

• The BALL is DEAD.
•  A STRIKE is given to the batter.
•  RUNNERS must RETURN to original bases occupied at the time of pitch.

In NFHS play: Intentionally preventing the ball from entering the strike zone and making a travesty of the game can result in either a warning for unsportsmanlike conduct, restriction to the bench, or even ejection based on the umpires judgment.

Rules Reference
NFHS 7-2-1h / ASA 7-4L / USSSA 7.5B / NSA 7-7j
PONY 8-6g / BABE 2.0 (Strike-E)

Many associations are adopting the use of a DOUBLE FIRST BASE to minimize collisions and reduce the chance of injuries at first base.

When using a DOUBLE FIRST BASE the following applies:

• The DEFENSE MUST use the WHITE portion and the BATTER-RUNNER MUST use the COLORED portion of the base when a PLAY is being made on the batter-runner.