Fernadito, when you are making a groove or dovetail (also a groove) the forces on each side of the bit oppose each other and equal each other. What James was referring to was where you would be working on an edge treatment. You would have most of the bit behind the face of your fence in that situation. You would never want to have the workpiece between the fence and the bit. That is trapping the workpiece.
There was a recent post by someone who had tried to use their router and table to cut a workpiece to the proper width because they did not own a table saw. The workpiece was thrown from the table with great force. That person was also probably feeding the workpiece from the wrong direction. As I already said, normal feed direction is from right to left, but that is with most of the bit behind the fence. In this case, feed direction would be from left to right because you are working on the other side of the bit. The rule for feed direction is that you should always be feeding toward the flat face of the cutter and never towards the beveled side. Feeding in the correct direction will require you to push your workpiece into the bit and going the wrong way the router will either drag you towards it or rip the workpiece out of your hands and throw it.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.