The good tuck

Ah, the tuck stitch. You gotta love it. Seriously. We machine knitters only have 4 basic "stitches" (knit, purl, tuck and miss). Two of them are actually the same stitch (knit and purl) seen from opposite sides. One of them (the miss, a/k/a slip or skip) isn't really a stitch, though knitting a stitch when you meant to miss the stitch would make a huge difference in most projects.

That leaves the tuck. It's made with the setting of a cam and the passing of the carriage. When the machine needle moves up enough to catch a loop of new yarn, but not far enough up for the old stitch to fall behind the latch, the tuck is made. After the carriage pass what's on the hook is the old stitch and a new loop of yarn. Not a fully formed stitch, the tuck is often referred to as a tuck loop. (See the blue tuck loop in the center of this animation from the Knowledge for Innovation website.) Since the old stitch remains in the hook while the adjacent stitches knit, the old stitch gets pulled and, by stealing a little yarn from the adjacent stitches, becomes elongated.

If you've made a fisherman's rib or half fisherman's rib on the machine, you've made many tucks. Needless to say, making repeated tucks and using the tuck in combination with other stitches is one of the major ways of developing stitch patterns. Taking another look at the animation, the single tuck loop isn't very noticeable when hidden behind knit stitches. You can see that in a jersey fabric, it's on the purl side that the tuck loop really shows.

Below are some tuck combinations on a rib fabric.

Yes, you can have tucks on adjacent needles. Click image to enlarge.

A tuck loop can be an awesome master of disguise. Tuck loops and racking combine to make the stitch pattern in this hat.

Right side

The inside of the hat

Hand knitted brioche patterns are made with the tuck. Hand knitters call it "knit 1 (or more) below".

When experimenting with the tuck loop on the machine, remember that depending on the thickness of your yarn, you can only tuck as many times as the hook of your needle will hold. Eventually you do have to knit the loops off the needle.