Vertical Reference (Longlining)
Longlining is slinging with a line over 75 feet long. To do this successfully, you have to maybe look downwards out of the door or a bubble window, and not through the front with a mirror, which is the more traditional method, hence its alternative name of Vertical Reference (having said that, in some machines, notably the Astar, it's quite difficult to look out of the door, and the weather often means you need the doors on, so using mirrors is sometimes the only way). The long line has a hook at the end, which is inside a metal brush cage, to both protect it and provide weight when you are flying around with only the line attached. As the extended hook needs an electrical supply, there will be an extra cable to control it, taped to the longline or incorporated directly into it. It will have a couple of spare feet at either end, which should be kept well away from the manual release. You may also find a couple of changes to the airframe, in the shape of bubble windows, or instruments being repeated to the side (don’t kick them on your way in) so you can see them, although you soon get used to the sound of machinery getting near to the overtorque stage (don’t ask!).

A metal line is made of two strands of wire rope, wound around each other, to help prevent spinning. Kevlar (i.e. Spectra) ones are extremely light by comparison, but just as strong, although they can stretch. The only real pain with them is ground handling, because they have a tendency to get tangled up into knots if they don't have a protective covering (one suggestion is to get a large bucket, place the hook in and loop the line around the inside). However, they can also pick up twigs, etc., and grease from drills is apparently not good for them -they can also snag on trees, as I found out once. Being light, they can be hard to punch off in those circumstances, and tricky to position for the crews in a wind when empty.
You need to be particularly careful not to kink any line, which means not driving over it, landing on it, dragging it along the ground, or dropping it from great heights.
Long-lining (or vertical reference) proficiency to some customers involves putting a small load at the end of a 100 ft line onto a 4 x 8 ft sheet of plywood three times from different approach angles, or even putting a load into a barrel.
The human eye is actually a very poor optical instrument (you would get a better picture from a pinhole camera), and it is your brain that is responsible for turning the image the right way up in the first place, and for resolving the many colours the eye is capable of distinguishing. What this boils down to is that above about 40 feet, it is very difficult to judge height properly, especially through your central vision (the weakest part), and handling depth perception needs some practice. It is for this reason that you should always look at the load through the same medium (preferably none at all) so you give yourself the best chance. For example, you are not helping yourself by looking at the load with one eye directly and the other through a panel. Having the Sun on the opposite side of the machine to you is a useful tactic as well, so you can use the load's shadow to tell its height from the ground better.