Consider the following BBN:
In the section on converging connection we saw that even soft evidence about A (Martin late) created a dependency between A's parents (B 'Martin oversleeps' and C 'Train delays'). Specifically, increased belief in A being true leads to an increase in our belief in the truth of both B and C. Now we also know that train delays can cause Norman to be late. Suppose then that, in addition to Martin being late, we also know that Norman is late. Since Norman never oversleeps, his lateness is explained by train delays. We therefore have hard evidence about C which explains why Martin was late. Because Martin's lateness has therefore been explained away, our belief in Martin oversleeping reverts back to its original value.