A ruined orgasm occurs when stimulation stops at the exact moment of inevitability - too late to prevent physical release, but without the stimulation to make it satisfying. The result is ejaculation without pleasure.
For the recipient, it's physically frustrating - the body releases but satisfaction is denied. This is used in orgasm control to maintain arousal and desperation while technically "allowing" orgasm.
The dominant must learn to read their submissive's responses precisely to time the removal correctly. Too early and orgasm is denied (different effect); too late and it's a normal orgasm.
In chastity or denial dynamics, ruined orgasms may be permitted while real orgasms are not - providing some release while maintaining the psychological state of denial.