Monogamish relationships are neither fully monogamous nor fully non-monogamous. The way it works is that, occasionally, the partners open the relationship up to another person, or multiple people. This is often a way of meeting sexual needs rather than for establishing an emotional connection with others.
This makes securely attached people more likely to feel emotionally secure and satisfied in their intimate relationships. They’re comfortable being in a couple, but also secure enough to be by themselves.