The top measuring slide is to get the first cut to length without wasting too much.
Clamp the moulding in the saw. Make your first cut. Slide it to the right until you can engage the measuring arm above the blade. You will need to move the second slide in or out to line up with the point of the miter.
Then slide the moulding until the desired length is met. And just slightly more, maybe 1/4 inch.
Then when you turn the stick to the right arm, you will have little waste after making the second cut.
I hope this makes sense.
I could show you faster than I can tell you.