There are two ways to encourage muscle growth. The way that most people think of growing their muscles involves actively contracting the muscle to accomplish movement of some kind. Picking something up off the ground, pushing something up into the air, pulling something towards you. Exercises like pushups and pullups require you to try and displace some sort of mass in order to grow the muscles. Sometimes that mass is your body, and sometimes it is a weight of some kind.
The other way to encourage muscle growth is by resisting the movement of a mass of some kind. Negative pullups are a great example of this. Instead of trying to pull your body up to a bar, which some people are incapable of doing, you instead start at the top and try to prevent your body from descending to the ground.
Liv’s comment about how she has trouble balancing on a swiss ball is a perfect example of passive resistance. When you do an exercise on a swiss ball, be it a bridge or a bench press, you are not only performing the exercise as you would on a normal bench or on the floor. You are also trying to balance yourself. The natural inclination when you are on a swiss ball is to roll to the side. You engage your obliques to prevent your body from twisting, and the fact that you are trying to prevent your torso from rolling onto the floor is a great exercise in and of itself. You are training your body to do more than one thing at a time, and you are causing your muscles to learn how to work together to accomplish a goal.