Southern California Tracon instructor controller described operational error event when Developmental Controller assigned altitude below MVA and transferred the aircraft to the next frequency.

Narrative

I was conducting on the job training to Developmental on Shore sector at Southern California Tracon in San Diego; CA. The MVA in the vicinity of Santiago Peak on the Kayoh arrival to SNA transitions from 6;000 to 5;100. Aircraft X at 8;000 requested lower at DEJAY intersection. The Developmental descended the aircraft to 6;000 where the MVA is 6;000. The Developmental then descended the aircraft to 5;100 before the aircraft departed the 6;000 MVA and switched frequencies to the Arrival Controller before I could stop the descent. The MSAW alert activated in the 6;000 MVA at an altitude of 5;400 in the previous sector's airspace on the next controller's frequency.The Developmental has had over two hundred hours of training at this position without any prior FAA experience. The precarious nature of the MVAs was also discussed numerous times with the Developmental. Apparently; this information was never understood by the developmental. Unfortunately; stories of this type are rampant throughout the agency. Since imposed work rules were forced upon the controller workforce in September 2006; the workforce has been decimated by retirements. Instead of experienced controllers in place; we have been forced to train replacements who often have no prior FAA experience. Math and verbal skills are lacking. It is really hard to be an air traffic controller if you are unable to speak clearly or do any rudimentary math. Remember; Southern California Tracon is the busiest approach control in the world. To be perfectly honest; I am surprised that I made it almost 3 years training people who should have started their careers at less busy facilities before an error of this type occurred. It is not fair to the developmental; it is not fair to the instructors; and it is especially not fair to the flying public. They deserve better. Screen the developmental before thousands of dollars are wasted and lives are endangered.