Spindle Load feedback in LinuxCNC

Hi, I was wondering if someone could teach me about the mechanics of spindle load and how it works?  Is there a special motor and drive required to get the load feedback data or is it generated by a formula comparing the RPM with a multimeter as some kind?  Or is the load just the amperage alone?  Can someone explain it and what all kind of components and wiring would I need to get a spindle load showing in LinuxCNC?  Thanks

Usually, an analog input is required. However, another option is to connect an RS232/RS485 interface to the spindle inverter. I chose the second option, and it works well. You can skip to the 1-minute mark in the video to see it in action. There's a small bug with the numbers, though

my solution is convert analog out of vfd ( config analog out is load ) to fequency + use mesa card counter function

Ok awesome! Nice setup. I wish I had a dialed machine setup like your running. Unfortunately I don't have an inverter though in my situation. I have a AC servo spindle. I wanted a VFD and BLDC but nothing would fit my machine and it needed to be running before my EOL LOL. Anyways my idea is if I could get a lesson on what's needed to make a readout like you've got and somehow program it so that if the load is too high it would alter the feed/speed so that a chip size of set value would never exceed the capabilities of the motor. That and another possible feature would be an estop trigger if the load was too high and RPM lower than a set threshold. Is any of this possible or am I just crazy? Anyways sorry for the weirdness. Thanks