Tutorial 3dsMax -- Flex Modifier
The Flex modifier simulates soft-body dynamics using virtual springs between an object's vertices. You can set the springs' stiffness, or how actively they keep vertices from coming close to each other, as well as stretch, or how far apart they can move. At its simplest, this system causes vertices to lag behind an object as it moves. At a more advanced level, you can also control the sway, or how much the spring angle can change.
Sometimes completely unknown, the modifiers are very useful during the normal workflow of 3dsMax. These are old videos created with 3dsMax2010 but the modifiers are equal to the latest versions.