ZME Controller witnessed a loss of separation event during a training session; the reporter expressing concern regarding the FAA's 'mitigate risk' philosophy; i.e. de-combining positions when traffic builds; and the effect it might have on a trainee's exposure to various volumes and complexities of traffic.

Narrative

Sectors were combined with a RADAR trainee working under the supervision of an OJTI. Air Carrier X; landing BNA via GHM 5 Arrival; [was] level at 11;000 FT. Aircraft Y; also landing BNA via GHM 5 Arrival; [was] descending to 11;000 FT. When Air Carrier X leveled at 11;000 FT his ground speed rapidly reduced; creating an overtake from behind by Aircraft Y. When Aircraft Y's mode C readout was 12;400 FT; the OJTI instructed Aircraft Y to maintain 12;000 FT. Aircraft Y replied he was already through 12;000 FT; but would go back up. The OJTI failed to provide positive separation in a timely manner. The FLM should have realized the complexity and volume of the traffic and split the sectors. Even though we are in a safety culture; my thinking is that a trainee needs to train with a high volume and complexity of traffic as often as possible. This needs to be done in order to hone his/her skills in case a time should arrive where that kind of situation arises and nothing can be done to alleviate it. The mindset nowadays is to split sectors as quickly as possible to 'mitigate risk;' but if we always do that then training will suffer tremendously and we will end up with a workforce with not nearly the skills that will be needed as the volume of traffic increases in the future as predicted.