Undermining means to erode the base or foundation. It also means to damage or weaken. This is true in surgery as well as in other things. When plastic surgeons undermine an area , we weaken or divide the connection between the skin and the body wall. It allows us to lift the skin and stretch it down. But the other part of the definition is also true. The more you divide and lift the skin, the more you divide the nerve and blood supply. If the nerve supply is out, you are numb. If the blood supply is out, the skin dies. So, where's the balance?Any thoughtful plastic surgeon will divide only that which has to be divided to get a good result. That's also why operating on smokers or those with other causes of poor circulation can be a really bad idea. That, by the way, was one of the big reasons that liposuction was often limited with abdominoplasty. The technique of lipo-abdominoplasty has shown us that it can all be done safely but with intentionally limited undermining.