Inverted flank is another cam profile term that you may not have heard of or understand. Like most camshaft terms it is used as a marketing tool to sell camshafts.
Inverted flank profiles are roller cam profiles that have a negative radius or concave shape on the side (flank) of the profile. Kind of like a dip in the road. Depending on the severity of the dip, it may not cause a problem or it may cause a big problem. These are only on roller profiles, not flat tappet profiles.
When lift is increased for a given cam profile, velocity and acceleration also increases. Duration will stay the same but the distance to travel is farther. This obviously causes the tappet to have to move faster. In a flat tappet profile the natural design tendency will end up with a smaller radius at the nose of the lobe. This is a limitation. The nose radius can only be so small. The velocity will also become too fast for the diameter of the tappet face. Another limitation. In a roller tappet profile the natural design tendency will end up with a negative radius in the flank (or flanks) of the profile. A limitation caused from the acceleration increasing.
There are techniques that can be used for both flat tappet and roller profiles that will help the designer control these limitations. Some designers use them, some do not. For those that do not their cam profiles will be limited in area or they end up being very rough and maybe damaging to the valve train.
The flat tappet profile limitations are pretty straight forward. You end up with a cam profile that has a sharp nose and runs off the edge of the tappet face if the limitations are ignored and not controlled. The roller tappet profile will eventually have a negative radius (the dip in the road) that is just too small to actually grind. The standard grinding wheel diameter on a camshaft grinder is 18-inches. That will have a 9-inch radius. It is possible to get smaller grinding wheels. Having ground camshafts for many years, I know this is a real pain to do. Grinding slightly inverted profiles is still a pain even with the standard 18-inch wheel. It really slows down the process and the camshafts usually cost more. The customer may not be too concerned about this, he wants to know if they will make more power. The answer is no. More times than not they are much harder on the valve train. Remember the dip in the road. A smooth road is much easier to drive on. Slightly inverted cam profiles may not be as hard on the valve train but they are still a pain to grind and there is no advantage in performance. Some of the camshaft marketing will try to tell you different. Using the proper software and techniques to design a non-inverted roller profile will always produce a better cam profile.