Below is one such example, and I must admit that as far as getting excited about pulling a calf, this is as close as I've ever been. Getting to pull one in the daylight, with lots of help present so I could take photos, isn't the usual story of how it goes.
Just to let you know, I'm going to show you everything. If you don't want to see everything involved in pulling a calf, now is the time to leave.
This is typically what it looks like when a heifer is calving in general, and often the stage she ends up at when she needs help, although the sac is often broke, and you can see one or two feet sticking out. One reason heifers sometimes have difficulties is because the calf is backwards, or in another awkward position. Just like in humans, most come the "right" way (feet and head first), but not every one. One of the first things you note when observing a heifer calving is how many feet are out, and if they are pointed up or down. You want to see two feet, toes pointed down, because that means the calf is coming the right way.
Another reason we might need to pull a calf is because the heifer is a little lazy, and just won't work at it hard enough. Or, sometimes the calve's head, shoulders, hips, or all three, are too big to pass through the birthing canal with ease, and assistance is required to help get the calf maneuvered and pulled out of the birthing canal. With modern advances in breeding, size related issues aren't as prevalent as in decades past, but it does still happen.
If you don't pull calves in these instances, the chance of death greatly increases for the calf, as does the chance of physical injury or health related problems for the cow. In some cases, the cow will also die. Best case scenario for a heifer that experiences calving difficulties, or dystocia, and doesn't receive assistance is a longer interval following calving for both mom and calf to return to optimum health. But, usually the turnout is much less optimistic than that.
We watch our heifers 24-7, and there is always one member of our family on-hand during calving to provide any assistance needed by these first-time mothers.
Then he uses a control (the read handle) to pull the rubber strap up, until it is snug under the heifer's flank. This is used to keep the heifer on her feet while pulling the calf, and keeps her back straight, which reduces the chance of her being injured. It also prevents the chance of her sitting down halfway through pulling her calf.