Txernyuk

These are formally known as Solder Sleeves. Their advantage is that they make a very reliable bond if used properly (most common mistake is not using enough heat), even waterproof if you opt for the environmental kind (with the glue rings at either end). They are wire gauge specific for *both* ends and sometimes are even intended for 2:1 splits.

That being said, soldering is typically frowned on in high vibration environments because mechanical strain that is normally distributed down the length of the cable is concentrated at the solder joint.

However, if your situation dictates you must solder:
-Slide the solder sleeve on and past the joint, onto one of the sides of wire
-180degree bend the exposed ends of each wire
-Hook these bends together and pinch them shut around each other
-Slide the solder sleeve back to the joint
•Position so that the solder ring is centered on the joint you just made
-Use a heat gun to shrink the solder sleeve on the joint
•You’ll know when it’s finished because the solder ring will liquify and flow into the wire
•The heat shrink body should be tight around each wires’ insulation
+Slight ooze/protrusion of the adhesive ring is a sign of a good bond

If you heat too much or too long it will either char or split entirely, resulting in you starting over.