You are probably already familiar with sliders through your skydiving experience. The slider exists to stage the opening of your canopy and henceforth make the openings softer. Practically all skydives are done with a slider.
In general, all freefall delays that are less than three or four seconds are done slider down. Beyond four seconds, the use of a slider becomes a necessity in order to soften the opening.
In skydiving, most sliders are standard sail sliders. A rectangular piece of solid fabric with a grommet in each corner. In BASE jumping, the need for more control over the canopies deployment sequence has lead to different types of sliders.
Nearly all variations on the sail slider are done to increase the opening speed compared to sail sliders. In skydiving the deployment sequence can easily take over five hundred feet. The altitude is available, leading to very comfortable openings. Because BASE jumpers generally open lower, their want their canopies to be open faster. They are willing to trade some comfort for the added safety of a quick opening. Obviously, an opening that is too hard can lead to injury too.
All these considerations have lead to contraptions like the beer-can slider (a sail slider with two holes the size of a beercan), the bra slider (a slider with two cup shapes) and the cross connector (a cross between the four risers). None of these ever became popular in either BASE or skydiving.
Today there are three major shapes of sliders left. In order of the decreasing opening speed using otherwise-identical sliders, they are the large-hole mesh slider, the small-hole (or marquisette) slider, and the sail slider.
Large Hole Mesh
The large hole mesh slider is a rectangular slider with a mesh instead of solid fabric. The holes of the mesh are larger than the marquisette or small hole mesh slider. When people refer to a mesh slider, they usually refer to a large hole mesh slider. For most people it is the BASE work horse, applicable to most jumps that require a slider, from five second subterminal delays to full terminals delays.
Small Hole Mesh
The small hole mesh slider is the same as the large hole mesh slider but it has finer holes in the mesh. Some manufacturers refer to this slider as a marquisette slider. Some canopy, jumper and packing-technique combinations can lead to hard openings when a large-hole mesh slider is taken to a terminal jump. A small-hole slider provides more air resistance as it slides down, slowing down the opening.
The sail slider is just like a slider in skydiving. You can put one on your BASE canopy if you make a skydive with it, but it has no use on actual BASE jumps. Although a sail slider might, in principle, be safe when used in combination with a conservative opening altitude on a terminal object, in practice the possibility that the jumper might have to deploy subterminal — and the extraordinarily unreliable heading in that scenario — makes the sail slider a very dangerous choice. In fact, a fully terminal BASE jump gives the same opening speed as a skydive, so why not just experience a large or small hole mesh slider and get used to the openings?
Choosing a slider type
When in doubt, err on the side of an opening that is too fast. Most canopies, jumped by a fit jumper using an otherwise-reasonable packjob, may yield hard — but not injurious — openings even using a large-hole mesh slider. It is better to have a sore back the next day, than to pound into the ground under a snivelling canopy. Even better; ask other people that are jumping the same canopy as you are. They might have some advice to control the opening speed.
Small hole mesh sliders should be avoided on subterminal jumps. They should be used on terminal jumps only after the jumper has found that a large hole slider leads to openings that are too hard.
The size and aspect ratio of of your slider can greatly affect the way your canopy opens and flies. You should consider yourself a test jumper if you plan on experimenting with slider sizes, and it is always recommend to take new sliders for some skydives first.
Generally it is the manufacturer that will release a new slider model for a particular canopy, and they will only do so after considerable testing.
To illustrate the complexities of slider sizing, try to consider what change is required to speed up an opening. An initial thought might be to reduce the size of the slider. This will reduce the air resistance as it comes down, allowing a faster descent, creating a faster opening. However, at low airspeeds it is less the air resistance and more the reefing effect that determines the opening speed. Because of this a second consideration is to enlarge the slider. This allows the canopy above it spread out more while the canopy is still at the top. The canopy will therefore catch more air, exerting more force on the line groups, pushing the slider down harder.
The point? Slider shaping is a complicated science and art-form. You can lengthen or widen it. You can change the surface area or the aspect ratio, and you can play with the size of the mesh holes. All factors determine your slider behavior and none should be taken lightly. As mentioned before, experimenting with sliders makes you a test jumper.
Slider Up on Short Delays
Interestingly enough, if you jump a slider on delays shorter than three seconds (for example from a forgiving bridge) the difference between large hole, small hole and sail sliders becomes barely noticable. Normally the difference comes from the air the slider displaces as it comes down, but during a low speed opening, there is very little air to displace. Of course the slider still functions as a reefing device as it comes down the lines, slowing the opening down that way, but it matters very little what kind of slider you actually jump.
The most important factor to consider when using a slider on short delays (intentional or unintentional) is heading. Heading performance becomes progressively worse as a slider is used on shorter and shorter delays. The cautious advice is never to use a slider of any kind on a delay of less than three seconds where there is a solid object behind the jumper.
The debate on how to orient a slider is an ongoing one. The considerations are the orientation of the grommets and the orientation of the tape. Some say the tape should be on top. This way the air resistance will push the mesh into the tape reinforcing it. Others say that the shape of the grommet determines it, meaning the rounder edge should be facing up or down, exerting less or more force on the lines.
As far as BASE WIKI knows, there has not been any incidents from sliders that were oriented the wrong way. As long as the longer side is facing front and back and the short sides to the side, the slider will most likely do its job. Caveat emptor.