Goku learned this during the five years of inactivity between the end of the Cell Game and before Gohan started school at Orange Star High School. As the name suggests, the major requirement for this type of fusion is to have the two people involved in the fusion perform a dance, which are mirror images of each other, resulting in a new warrior, who has the attributes of the two warriors who formed the new one. When performed successfully, the newly formed warrior will be significantly stronger. There are other requirements and conditions, of course. The two fighters must be similar in size, build, and power (especially power; they have to be almost exactly the same, but this isn't a major problem, since most of the Goku-tachi can control their power levels, so the stronger fighter just needs to lower his/her power to match that of the weaker). The dance must be performed correctly for a successful fusion. Any minor mistake in the dance can result in a defective fusion. The fusion will last for 30 minutes, by which time both fighters will return to their normal selves, but there doesn't appear to be a way to end a fusion before this 30 minute time limit. After a fusion ends, the two fighters must have a waiting/resting time of one hour before the fusion dance can be performed again.