2.6 In a special situation like a 4th player getting involved in the attack (mostly the GK) the 3-men-def-rule will be not applied, because it will turn into a 3vs4 situation. Once the gk is offering himself for a pass in the attacking zone, it turns into 3vs4. In this case the opponent's striker is allowed to help in defence to get an equal number for both sides.
i have no idea who is the author of this rule but it's the dumbest thing i've ever read. we play 3-men-def-rule, right? attacking with 4 players is a risk, that's why few teams do it, we should be allowed to take that risk to have higher chances to either score a goal or concede one. this rule doesn't give us that opportunity, we can't call 3def a 3def if striker is allowed to actually defend. i've been playing on 3def for years, it was always like that, everyone knows that, just remove that stupid rule and we're all happy.