He is referring to the INS spring.
It replaces the factory trigger spring that should be replaced every 5k.. Honestly, doing that is not a big deal.
You must have a all metal trigger to use this drop in.. The newer polymer covered ones will not work with it.
It's the first item listed on this page:
http://www.gunsprings.com/Semi-Auto Pistols/BERETTA/92, 96, AND CENTURION/cID1/mID2/dID36
I have a factory D spring in all my 92s. It cuts the SA pull by about 1 lb and the DA pull by 3-3.5lbs. The standard strength INS spring adds weight back to the trigger. When I tried it - it actual put the trigger almost back to factory weight.
I decided to sell it and just go back to the factory spring. There is a reduced weight INS spring - it works for some... And, for others, it seems to make the trigger be stagy and have trouble returning back to the forward position. There seems to be some variation in specimen to specimen in that regard.