"He also who is slack in his work is brother to him who destroys."
The word slack is the Hebrew word rapeh and means to sink down, a weakening of the hands, even a dropping or less than full force.
Solomon is telling us a correlation that we would not readily make. The one who gives a minimal effort is related to the person who destroys because destruction will result from giving less than the best. There is a family resemblance between minimal effort and being a destroyer. In other words, doing a half-hearted job – doing only what will get you by but not what should be done – ends up being destructive.
Notice that the proverb says that the person who is slack is brother to the one who destroys. In other words, they are related but not the same thing. They are family members.
Solomon wants us to see these insights so that when we are hiring people or picking people to hang out with, we will be wise. Don't pick the people who are minimalists who rarely, if ever, put out any real effort. While it may seem that they are actually doing work, they really are related to the person who smashes things up. Their minimalist attitude and work are very much like one who destroys. It could be because what they do usually needs to be done again or that it will not stand up to the rigors of pressure when the time comes or that it will fall apart of its own flimsy construction.
The key idea is do not hang out with the people who are slack or minimalists and certainly do not become one because you join a destructive family.
Many young people think or say things like, "Why should I work hard on school projects or why should I do work that my parents want me to do when I really don't care about that stuff?" The answer is because you do not want to become a minimalist. If you are doing it, then do it well. Do not train yourself to be a minimalist thinking that you will be able to dial up the effort and ability when something you really like comes along. Throw yourself into whatever is in front of you and you will be in the wisdom and excellence family. If you are a minimalist and a barely-doing-a-thing person, then you will be lumped with the unpromotable and destructive people who have a hard and sad time of this life.