Just to cite the rule:
"When an infielder intentionally drops a fair fly ball, including a line drive or
a bunt, which can be caught with ordinary effort with first, first and second,
first and third or first, second and third bases occupied with fewer than two
outs.
RULE 8 - BATTER-RUNNER AND RUNNER
Rule 8, Section 2
77
EFFECT: Dead ball, runners must return to the last base touched at the time
of the pitch. If an infield fly is ruled, it has precedence over an intentionally
dropped ball."
The other part part NCASAUmp referred to is the Rules Supplement definition of an Intentionally dropped ball:
"30. INTENTIONALLY DROPPED BALL.
The ball cannot be intentionally dropped unless the fielder has actually
caught it, and then drops it. Merely guiding the ball to the ground is not an
intentionally dropped ball."