Toe-End Gauge

[rfitzpatrick]

I'v the 'toe-end gauge' for my PicUp, it's '31. I don't understand the concept . What causes the toe-in to change? When I follow the instructions, it's a 'push-pull' adjustment (from my point of view), but all the attach points are secured with cotter pins. So how did the adjustment change? What happen? Are they that easy to get out of alignment? I do have new king-pins.

Now I'm having a very rough day -- plz be kind/thoughtful, I'm confused too