Enroute Controller providing OJT described a loss of separation event when failing to note a conflicting descent clearance issued by the developmental; noting the documentation of performance as a causal factor.

Narrative

RADAR training was in progress. An A320 was at FL330; southeast bound. LOA requires arrivals at FL290. An E145 was enroute at FL320. Trainee initiated a hand off on the E145 to an incorrect sector. The controller of that sector called trainee to advise of the error. The instructor looked down at note pad and was recording the discrepancy. During that time; trainee cleared the A320 to FL290. Data Block overlap was a problem; as the Data Block of the E145 was obscured by other Data Blocks. The on-duty FLM was standing behind the trainee; observing the sector operations. He caught the descent clearance and the conflicting traffic. He alerted the trainee and instructed the trainee to void the descent clearance. Trainee failed to respond. Instructor reacted to supervisor and attempted to stop the descent but the aircraft had already left FL330 and come into conflict with the E145. Recommendation; it is an unfortunate reality of OJT that occasionally; an error cannot be recalled. What is fortunate in this case is that the two aircraft in question were not in danger of an actual collision. I do not believe there is a better or more effective way to learn how to do this job. A computer simulation cannot equal live traffic. As an instructor; I do my best to ensure safety while still allowing my trainees to experience learning opportunities. Also; as an instructor; I do my best to accurately record anything notable ASAP; in order to ensure descriptive and meaningful training reports. When this error occurred; my eyes were averted from the scope and I missed the clearance that was issued.