How does straight-rail billiards limit scoring in the corner areas?

In straight-rail billiards, scoring in the corner areas is limited by the use of a "crotch," which is a small triangle marked in each corner of the table.1

When both object balls are positioned within this crotch, a player is restricted to scoring only three caroms. Upon the third carom, the player must drive at least one of the object balls out of the crotch area; failure to do so results in the loss of the turn at play. This rule prevents players from indefinitely scoring through repetitive, simple shots in the corner.12