How do ‘implants’ act as an effective method of contraception

How do ‘implants’ act as an effective method of contraception in human females? Mention its one advantage over contraceptive pills.

Most common form is a kit consisting high dose of birth control pills. These, if taken within 72 hours of coitus have been very effective as emergency contraceptives as they could avoid possible pregnancy due to rape or casual unprotected intercourse. Their side effects are menstrual irregulation, vomiting, etc., e.g. i-pill, pill 72, unwanted 72.