Undercuts are those portions of the part that can not be pulled in the line of draw. If an undercut were to be machined into a mold without a mechanism to releive it, the part would be destroyed upon mold opening or ejection.
Undercuts can usually be divided into two categories, internal and external.
As a general rule SLIDES are used to pull external undercuts and LIFTERS to pull internal undercuts.
There are many different types and styles of slides and lifters. Every tool shop seems to have "the only way that it can be done" and no two do it the same. Go figure. If the mechanism is durable and molds good parts then it is a good one. Place your trust in mold shops who have to service and repair their own molds on their own dime on this one.
This is another area where standardization is being attempted by several companies. The broad range of demands made by widely varying plastic parts makes this a difficult item to standardize however. I like the modular slides made by Omni.