Witnessed something similar myself in the flesh last year in Germany. Chance of a fast counter attack, was two on one. The sole defender (ex-Boro hatchet man Manny Pogatetz, incidentally) comes across and gives the attacker a nudge to put him off balance. Would have ordinarily been a yellow for a tactical foul (or even a red to be honest, but it was a long way out near the halfway line*). However, the ball squirmed away to the attacker's team mate who then continued the counter attack (advantage played)...he then put it just past the post and it went out for a goal kick IIRC.
However...Manny was not given a yellow as the "tactical foul" bit didn't "work" (as the tactic of fouling him, ergo a yellow card offence, "taking one for the team" etc. didn't come off).
If however the same situation occurred and Manny had hatcheted his leg off at the thigh then the situation would be the same BUT a red would then be shown for serious foul play.
That's how I understood it and how the debate online panned out at any rate.
* There is no such rule as "he was the last man" either, incidentally...that's another term that isn't in the laws.