I have seen the flush trims that are either flat or normal if you will then they have some that are up-cut or down-cut. What is the main purpose with these? what makes them so special compared to a normal flush trim?
The key to spiral bits is in the way they handle the waste material. Flush trim bits are intended to flatten an exposed edge of a board. The bits usually have a roller matching the diameter of the bit above or below the bit and use an edge guide or template to trace the profile of the edge on the work board. Spiral up-cut and down-cut bits are used primarily in plunge conditions away from the edges of the project board. They differ in the direction of their spiral twist and how they handle the waste. Up-cut bits move the material out of the wood or up towards the router bit, while down-cut bits move the material in the opposite direction. For that reason waste tends to clog up the operational area of the wood more. So why have the options?
First consider the orientation of your router. In hand held operations the router is oriented with the bit at the bottom. If the board had a clock face and the router is lowered onto the work piece, the bit turns clockwise. The up-cut spiral works to lift the wood disturbed by the rotating bit to turn with the rotation and lift the waste up and out. The opposite is true for down-cut bits.
Up-cuts are desirable for plunging deeper into a piece of wood. The waste material is lifted out of the work area preventing clogging which can actually break a bit in a deep plunge. Unfortunately, the lifting action of the bit will also cause fraying at the edges which can be unsightly. Up-cut bits are often used for cutting mortises and the edges of the opening are usually covered by shoulders on the tenon piece so the fraying is not noticeable.
Down-cut bits force the material down and create a much cleaner edge. Since they are not removing the waste it is necessary to move much slower with shallow cuts and frequent stops to remove accumulated waste. Down-cut bits are more useful for shallow dadoes or rabbets, exposed slots or grooves.