Swivels are used to keep your line from twisting. As you fish, the bait often causes your line to twist, and a swivel will prevent that from happening. Some baits don't twist as much as others, so you might be able to get by without a swivel for certain ones.
Look for quality swivels that spin easily. A barrel swivel can be made very simply and they are cheap, but higher-quality ones have a ball bearing in them and turn with very little pressure. They're usually stronger than the barrel swivel, too. Use them when fishing for hard-fighting fish, when trolling, or for any type of fishing when the bait spins a lot.
Use a black or dark swivel when fishing for species of fish that like to attack shiny objects. A bright silver swivel a foot up the line from the bait may attract the strike of a barracuda and not only will you not hook them, they will cut your line. You must use a swivel with some baits to keep your line from twisting.
A swivel can also be used to stop the lead and keep it away from the bait. This is the typical setup on a Carolina rig used for bass fishing. You thread a Bullet Weight on your main line and tie a swivel to it, then attach a leader from a few inches to several feet long. This keeps the bait away from the lead and can be used for live bait as well as plastic worms.
Regular swivels have a loop at each end to attach line to, but you can also buy three-way swivels that have three loops for the line. These swivels work well for bait walker rigs and any other time you need to tie more than one leader to a swivel. They're often not as strong as a regular swivel so make sure you get a quality three-way swivel if fishing for big fish.
Fish Finder Rigs
A fish finder rig is a good rig for catching fish that feed on or near the bottom in salt water and fresh water. This rig combines a swivel, lead, and hook to efficiently get your bait to the bottom and keep it in position without interfering with its action. For that reason, it works well with live bait as well as cut and prepared bait, and will work in still and flowing waters.
Start by tying a heavy swivel to the end of your main line. To the other eye of the swivel tie on a lighter leader twelve to thirty-six inches long, depending on how high you want your bait off the bottom (a lighter leader is a piece of line that is lower-pound test than the main line). Attach a Bell sinker to the end of this leader. Then tie another leader to the same eye if you're using a single swivel, or to the third eye if your using a three-way swivel. Attach your hook to this leader and bait it up.
What's a rig?
As the word is used here, it's a general term for specially prepared terminal tackle, but it can also refer to an outfitted boat.
The hook leader can be any length you need it to be to allow the bait to work freely. If you want it near the bottom, make is as long or longer than the sinker leader. If you want to keep it up off the bottom tie it shorter than the sinker leader. As the sinker sits on the bottom or drags along, the bait will follow behind it slightly above it, depending on how long you made the hook leader.
This rig is great for drift-fishing or still-fishing. And if you have problems with the sinker hanging up, the lighter leader should break and allow you to reel in your hook and bait or any fish that hit. You can also use an Egg sinker on the end and either peg it with a toothpick or crimp on a Split Shot below it, rather than tying it to the leader. That way it will slide off the line when hung rather than break the leader. You can quickly slip another sinker on the end and be fishing again.
If your lead slips off too easily when holding it on with a toothpick, tie an Overhand knot in the line below the sinker before sticking the toothpick into the hole in the sinker the line runs through. This will make is slightly harder to pull off but not as hard as if a Split Shot is crimped on the line.
You can also add a float to a fish finder rig to help make it work better. When drifting over a flat bottom the float can keep the lead just bumping the bottom. If casting, use a slip bobber (a bobber with a hole through the center that allows the line to move freely) and you can detect bites. It will also help you reel in your rig because it pulls the hook and lead up to the surface and away from trash on the bottom that would hang it up.