Definitions of Terms:
The Umpire should find that a legal catch has occurred pursuant to Definitions of Terms, “Catch,” or valid force out or tag has occurred pursuant to Definitions of Terms, “Tag,” if the fielder had complete control over the ball in his glove, and drops the ball after intentionally opening his glove to make the transfer to his throwing hand. A legal catch does not require that the fielder secure possession or control of the ball in his throwing hand when making the transfer.
A fielder may not jump over any fence, railing, or rope marking the limits of the playing field in order to catch the ball. A fielder may (1) reach over such fence, railing, or rope to make a catch; (2) fall over the same after completing the catch; (3) jump on top of a railing or fence marking the boundary of the field to make a catch; or (4) climb onto a fence or on a field canvas and catch the ball. In all four cases the catch would be legal, as dictated by the best judgment of the umpire.
The same restrictions apply to a foul ball descending into a stand. A catcher or fielder may not jump into a stand to catch such a ball, but reaching into the stand and making the play is permitted.
Play: Bases loaded, one out. Fielder catches fly ball and momentum carries him into the stands. Fielder remains standing. Can the fielder throw for a play?
Ruling: No. “Time” is called and all runners advance one base.
As provided in Official Baseball Rule 5.09(a)(1) Comment, no fielder may step into any out-of-play area to make a catch. However, if a fielder, after making a legal catch, steps or falls into any out-of-play area at any point while in possession of the ball, the base runners shall be entitled to advance one base and the ball shall be dead.