Reducing means removing water
A simmering sauce loses water as steam. The sugar and soy solids stay behind. So the sauce gets sweeter, saltier, and thicker together.
Sugar makes the shine
Thick sugar syrup lies flat and smooth. A smooth surface reflects light, so it looks glossy. Thin sauce soaks in and looks dull.
Read the bubbles
Watery sauce makes small, quick bubbles. Thick sauce makes big, lazy ones. The switch tells you the glaze is near.
Stop a little early
Syrup keeps thickening as it cools. A glaze can look right in the hot pan. Later it can turn hard. Stop while it still flows.
Take the pan off the heat for 10 seconds and look again. Hot sauce always looks thinner than it is.