The primary purpose of karezza is the maintenance, and indeed intensification, of desire within the context of long term relationships. According to Stockham, it takes two weeks to a month for the body to recover from ejaculation. If ejaculation is experienced more frequently, the effect is to 'drain the basin' before it has been replenished. This, in turn, induces feelings of irritation and rejection of the lover, as the body seeks to prevent further ejaculation. However, and arguably especially in modern Western culture where 'more is better', people often pursue orgasms, and therefore ejaculations as a way of trying to overcome those feelings, thus compounding the problem. The result is that over time - and reportedly within two to four years - the 'honeymoon of desire' is over, leading to substantial change in the nature of the relationship. Stockham's advocacy was that this same 'honeymoon period' could be maintained in perpetuity through limiting the frequency of ejaculation.
Once love making is no longer undertaken as a means to orgasm, Stockham suggested, the nature and purpose of love making changes dramatically, being focused on communication and physical communion. Orgasm should, she suggested, occur when 'the basin' overflows, perhaps once a month and no more than once a fortnight.
Coitus reservatus is not a reliable form of preventing a sexually transmitted infection, as the penis leaks pre-ejaculate which may contain all of the same infectious viral particles and bacteria as the actual semen. Beyond that, it is also unreliable for contraception, even if ejaculation is successfully avoided, because pre-ejaculate may contain sperm. The method is also unreliable because of the difficulty of controlling ejaculation.
Another difficulty of this method is that, if the man begins to orgasm (before ejaculation proper begins), the muscles can tense tightly in the lower body, such as legs and buttocks, causing the removal of the penis to be difficult. This could cause some ejaculate to enter his partner.
http://en.wikipedia.org/wiki/Coitus_reservatus