Inserting a needle into the rounded heart shaped marker enables you to smoothly pass the marker from one needle to the other. These markers are elastic, hard to slip off, and small yet easy to pick up. You can place a stitch marker when casting on or picking up, to count the stitches/rows. Simply slip the stitch marker onto the receiving needle at the location to mark. When circular knitting, placing a marker next to the first stitch of a row makes it easier to count off the rows.
These stitch markers come in an array of colours. You can colour-code place holders when knitting patterns.