Carl Bergstrom

An isometric can look like standing still. A position, a timer, and a mind somewhere else. That version builds very little, because the stimulus comes from the effort, not the position.
There are two kinds. In a yielding isometric I hold a load and stop it from moving me. In an overcoming isometric I push into something that will not move and try to move it anyway. The joint stays still in both, but the tendon does not. The muscle shortens and stretches the tendon, so it is under real load the whole time.
What changes the result is intent. A review of 54 training studies found that fast force only improved when athletes moved fast or tried to. So against a wall my cue is to try to break it. Under a load it is to own the position. With body weight alone it is to squeeze and recruit on purpose.