the flush trim bit has a bearing to run along your jig (a piece of wood that sits under your acrylic (plexi glass) you clamp them together and clamp them down on a table and pretty much trace the wood out with your router). just make sure that the bearing rolls on the wood and the bit cuts the your acrylic,(check the depthon the router bit). This eliminates the need to hand finish the edges and gives you a perfect clean cut. if you cut a lot of plastic buy a good flush trim bit.
Petersen Bros. Plastics in Chicago sells cutting knives which are much more efficient then using razor blades or utility knives. If you can get the blue handle (thick blade) and yellow handle (narrow blade) and use them in sequence you get the best cut for scoring and snapping acrylic.