[SanDiegoSteve]

From Jim Evans:

6.05(m) A batter is out when a preceding runner shall, in the umpire's judgment, intentionally interfere with a fielder who is attempting to catch a thrown ball or to throw a ball in an attempt to complete any play.

Official Notes - Case Book - Comments: The objective of this rule is to penalize the offensive team for deliberate, unwarranted, unsportsmanlike action by the runner in leaving the baseline for the obvious purpose of crashing the pivot man on a double play, rather than trying to reach the base. Obviously, this is an umpire's judgment play.

Cross References: 2.00 Offensive Interference, 7.08(b), 7.09(f, l), Appendix 14

Historical Notes: The rules committee which recodified the Official Rules in 1950 felt that runners were taking unnecessary liberties in crashing into the pivot man at second base. To stem this unsportsmanlike action, they created Rule 6.05(m). This was one of their most significant amendments that year, and it gave umpires a specific rule to enforce. Shortly thereafter, the case book note (circa 1954) was added which elucidated the intent of the rule even further.

In 1963, the definition of offensive interference was expanded and provided a clear explanation of the enforcement principle of this particular rule.
Professional Notes: The runner should be declared out if he deviates from a direct line to the base and subsequently interferes with the fielder making or completing any play. Traditionally, runners are allowed to contact or collide with the defensive player at second just as they are on plays at home plate. However, different guidelines exist:

(1) The runner may divert his path in order to crash the pivot man but he must be able to reach the base with some part of his body;

(2) The roll block is illegal. The runner must not leave the ground and contact the fielder. If, however, he hits the ground first he is allowed to crash into the pivot man provided he does so at the base; and

(3) The runner may slide through and beyond the base toward left field and be unable to reach the base provided that he does not do so in order to contact the fielder who has retreated to this position off the base to complete the play. In that event, the previous guideline is in effect and the runner must be able to reach the base with some part of his body.

The American League regulations offer the following guidelines:

A runner, who in the opinion of the umpire contacts or attempts to make contact with a fielder with a slide or roll block that is not a bona fide effort to reach and stay on a base, may be called out for interference and when appropriate a double play may be called. Any definite change in direction by the runner to contact the fielder would be considered interference.
If a runner hits the dirt, slides and rolls, it does not constitute a rolling block unless he leaves the ground and makes contact with the fielder before he slides on the ground. If the initial contact is with the fielder instead of the ground for the purpose of breaking up a double play, it will be a rolling block.

The above are merely guidelines for the umpires in making their judgment calls.

Also, I have found this tidbit of information, though I cannot ascertain the original source:

Beginning with the 2004 season, if a runner makes any contact that begins after the bag, on the shortstop side of second base, the batter-runner will also be ruled out.