Here is the rule as reinforced by Avalon Hill.
The Tiger immediately disrupts a soldier unit upon entering the hex (face-up counter). This happens before defensive force occurs in that hex. Units with face-up disrupted counters cannot perform defensive fire.
So, in order to stop the Tiger tank from using its special ability (remember- special abilities trump the rules, as explained at the back of the starter set rule book), a soldier unit must employ its defensive fire while the Tiger is in the adjacent hex. Most soldier units don’t have enough attack dice to equal the Tiger’s frontal defense of 7, so the Tiger will have no problem moving into the hex.