The phrase "to let off" means to release or discharge something, such as steam from a boiler, water from a hose, or anger from oneself. Full definition
Secondly: don't let off steam when your child does, getting angry with your child when he hits only proves he can use aggression to have power over you. (free-toddlers-activity-and-discipline-guide.com)
I could be talking about real life, because that's essentially what it is, but no, I'm not letting you off with experiencing the mundane. (futurebetagamer.com)
Some dogs when let off leash will simply run away. (perfectpaws.com)