@klafkid’s point is that one axis will reset as soon as you hold down Shift and drag, so the minimal unwanted movement is then always reset to 0. In the gif below, I clearly start out dragging up and to the right, affecting both the X and Y axis a tiny amount, but then when I hold down Shift and drag, the Y axis resets to 0.
Even though I started dragging in a way that would affect both X and Y, you can see the Y is still at 0 when I’m done and only the X has been changed.