The history of the locks and holds of aikido, like many of the jujitsu schools are rooted in the sword arts of the Japanese samurai. Aikido locks and holds were originally intended to render an opponent immobile for a short period of time prior to finishing them off with a final coup de gra using a sword or other weapons. Today the locks and hold are also readily using by law enforcement personnel as a tool for non-lethal immobilisation's.
Learning the locks and holds takes time, as does the art of aikido. They are best learnt with an initially cooperative partner so that a feeling for the joint manipulations and the physical connection to the rest of the both can be learnt and felt, after this use of the techniques as an effective tool begins to develop.
Aikido techniques consist of a strong entry into the opponent (partner), controlling of or joining the movement of their centre (momentum ) before finally finishing them off with a throw or a pin based on a joint lock.
One of the most common traps is that the lock becomes the focus of the technique before the proper entry, balance breaking and control is established.
While there are many aikido locks and pins, the most common of them use a very simple nomenclature 1st technique, 2nd technique etc... of course these are named in Japanese and sound much more mysterious as ikkyo, nikkyo, sankyo, yonkyo.
Some serious work has been done on examining these locks closely including one of aikido's most painful lock Nikkyo.
Eckert,J.W and Lee, Ta-Kwong "The anatomy of nikyo (aikido's second teaching)", Perceptual and motor skills v77 pp707-715 1993
Olson, G.D.,Seitz,F.C, Guldbrandsen, F. "An anatomical analysis of aikido's third teaching: an investigation of sankyo", Perceptual and motor skills, V78, pp1347-1352, 1994
Hidden also in the locks and pins are the use of pressure points to aid in the effectiveness of them. I think its a mistake to go for preassure points on their own as they are fraught with problems, but they can be an unexpected bonus. See Pressure points in Aikido