With so many head-first sliding runners in baseball there are more fielders who will drop a knee in front of the base to keep the fielder from touching the bag. Here, however, we are not talking about the home plate collision rule. In this case, we are discussing a play at any base where the fielder is attempting to keep a runner away from the base while making a play on that runner.
Basically, the rule states that if a fielder is in the act of making a play at a base and he is in possession of the ball or awaiting a thrown ball, he may block the base. This becomes a judgment call for the umpire to determine whether the fielder is actually fielding the ball or blocking the base too soon (before the throw is made).
Read this Insider Report, a case study of a play by the Blue Jays’ Josh Donaldson, who blocks Brad Miller from getting to third base. Rich Marazzi covers the rule and offers some excellent coaching tips.
6.01 (h) (7.06): Obstruction
The interpretation at the high school level is different from OBR. NFHS rules require the fielder to be in possession of the ball before attempting to block the base. The call for violating this rule is obstruction.
NFHS 2-23-3: The fielder without possession of the ball denies access to the base the runner is attempting to achieve.
MLB Umpire Greg Gibson and Chris Welsh discuss how a fielder may legally block a base in this video.