You DO NOT use them on top of your ruler, you line up the ruler on the fabric to the correct size you want to cut, slide the slide lock tightly next to it and then remove the ruler.
That is the original purpose of the Slidelock to be on top of your ruler. Not on the cutting edge of the ruler but to hold your ruler in place. It use to be called
RULE STEADY. I have one of the first ones from Australia and one of the newer ones. They are both exactly alike.