With the Oakland Athletics calling up Pat Venditte, he will be the first regular switch-pitcher to appear in a major league game since 1894, according to Chris Jones writing for ESPN.com. Venditte's first opponents, the Boston Red Sox, have three switch-hitters on their active roster: third baseman Pablo Sandoval, catcher Blake Swihart, and catcher Sandy Leon. What will happen when any of these fellows face Venditte this weekend?

2015 Official Baseball Rule 5.07(f) (formerly 2014 OBR 8.01(f)) covers this situation:

A pitcher must indicate visually to the umpire-in-chief, the batter and any runners the hand with which he intends to pitch, which may be done by wearing his glove on the other hand while touching the pitcher's plate. The pitcher is not permitted to pitch with the other hand until the batter is retired, the batter becomes a runner, the inning ends, the batter is substituted for by a pinch-hitter or the pitcher incurs an injury. In the event a pitcher switches pitching hands during an at-bat because he has suffered an injury, the pitcher may not, for the remainder of the game, pitch with the hand from which he has switched. The pitcher shall not be given the opportunity to throw any preparatory pitches after switching pitching hands. Any change of pitching hands must be indicated clearly to the umpire-in-chief.

In sum, when a switch-hitter comes to the plate, Venditte must first indicate to the plate umpire with which hand he intends to throw, and then the batter decides from which box he intends to hit. Venditte cannot change hands in the middle of a plate appearance unless an injury prevents him from throwing from that side.

From Jones's story for ESPN.com:

Because Venditte can throw with either hand, he can always hold the advantage, even, in some ways, against switch-hitters. He can push them to their weaker side, or force them to hit into the wind or toward the longer field of a ballpark, shorting out their power. If there's a man on first, he might choose to pitch left-handed to improve his potential for a pickoff. "The percentages are always in his favor," says Don Schulze, Venditte's pitching coach with the [Nashville] Sounds. "As long as he's in command, the house is always winning."

This rule did not exist before this confrontation in Short Season-A against Brooklyn Cyclones switch-hitter Ralph Henriquez on June 19, 2008:

From Vincent M. Mallozzi of The New York Times:

In the midst of those discussions, Venditte tossed warm-up pitches — with both arms. "I don't think the umpires really knew how to handle it," Venditte said. "It's not something you see every day." After a seven-minute delay, Smith ordered Henriquez to step into the box as a right-handed batter, and Venditte, now pitching right-handed, proceeded to strike him out, swinging. When asked before Friday's game if he had ever seen anything like it before, McMahon paused before uttering softly, "Uh, no."

Major League Baseball's rules committee determined that they wanted the opposite of what occurred in Brooklyn to be the standard, that the pitcher declares first, and then the batter steps in.

Throughout Cactus League play, A's coaches were careful to point out to umpires Venditte's arrival to the mound and remind them of the rule that there has never been occasion to use at the major league level.