The basic definition of amputation is "to cut off a projecting body part." Typically, this desire or fetish for the voluntary removal of limbs or digits is an all-consuming drive dating back to childhood. While amputation is arguably psychotic in nature, having the amputation done "cures" the problem in many cases, and the individual becomes happier and more well-adjusted. Many doctors are beginning to recognize the valid therapeutic value of amputation in a small number of cases. The desire for amputation can be similar to transsexualism, where the internal notion of self is not aligned with the external notion.
"Healthy" amputation drives tend to be very specific. The individual will have an exact notion of what parts need to come off (no more, no less), although this is not a universal truth. Most individuals do not sexualize their amputation drives, although some also find the modifications deeply erotic (and there is also acrotomophilia, a sexual desire for amputees).
In addition, this type of procedure is obviously dangerous, and no reputable doctor will perform it. The few that would most likely will face enormous backlash from their peers, thus leading to a high probability of the physician in question to rapidly cease such practices. Only a very small number of difficult-to-contact underground practitioners will perform this procedure, so most individuals who desire amputation may damage their own limbs beyond repair at great risk to their own lives to facilitate necessary removal. Luckily, hospitals rarely question the "accident" excuse.
Methods commonly employed involve everything from, "I chopped it off while preparing dinner," to inducing ulcers and infections requiring amputation, to guillotines. More desperate individuals may do things as severe as simply blowing off the offending body part with a shotgun, risking death in the process.