I have owned many products over the years designed to keep shirts tucked in including shirt stays and the rubber belt.  It is a never ending problem especially for those who wear a duty belt.  This is the best product I have found by far.  First attach it to the shirt just below the waist and tighten.  I find it easier to do one side at a time, stick one hand between the belt buckle and shirt as you tighten each end keeps it from bunching up around the buckle.  Just pull the pants up and hook the waist band on, then button and buckle outer belt. That's it...nothing will move until you take it off.  Shirt stays in, pants stay up and even a heavy duty belt won't drag them down.

I do agree it will fray the shirt over time but that should be expected, you are attaching Velcro to fabric.  What I do not understand are the complaints, the damage is on the tucked in area of the shirt tail which isn't visible.  It is a non issue for me.  Very pleased with the product.