Given the name "slack" I think each person is resting, what you see as the testes being instead a head. They are leaning up, against the back of the next person

Given the name "slack" I think each person is resting, what you see as the testes being instead a head. They are leaning up, against the back of the next person, rotated 90 degrees. This signifies that Slack keeps you connected, and leaning on each other, the connectivity indeed making life seem easier, as though you were slacking off.