shikaku, not shikkaku
Shi means dead and Kaku means angle. Hence, Shikaku refers to all blind spots.
The purpose of karate is knock-out.
Delivering a technique to a live target is difficult so the better your strategy - the better the odds.
Attacks that come from Shikaku have more potential to successfully hit the opponent.
You have to catch your opponent off guard. If he is on guard, you need to distract him or tactically manipulate him to create Shikaku.
The other one, Shikkaku means to be disqualified (the double letter means a slight pause as you pronounce it, like tettsui).