A grip is considered forward when the blade opposes the little finger. The Filipino grip is a forward grip technique where he thumb is not wrapped around the knife handle, like the hammer grip or along the back of the handle like the saber grip or along the side of the blade like the modified saber grip, but along the spine of the knife back. The idea is that the thumb guides, aims, and applies pressure to the spine of the knife, thus is able to increase pressure at the point of the knife blade. In effect, the blade can serve as an extension of the thumb, and for some it seems more natural.
The problems are several with this type of grip technique. First, it is probably better served on a small knife where the thumb extension feeling and association is more compact and reasonable. On a smaller knife, it can be a more comfortable and secure grip technique, but on a larger, heavier, and longer-bladed combat tactical knife, the thumb on spine is unwieldy and even uncomfortable. This is because it is unnatural for the thumb to be hyper-extended in a thumbs-up position, and the extension can mean that the thumb itself and its musculature and tendons are not protected and are subject to injury. Imagine a heavy strike applied to the main cutting edge of the knife in the photo. The thumb would be forced back toward the wrist, straining it. There is a reason that most martial arts systems teach a locked-wrist technique of impacts, and that is to protect this complicated wrist-hand-thumb joint. The second issue is that with the thumb extended, the motion of the forearm is more restricted. If you don't believe this try this simple exercise: with your hand closed in a fist, extend and rotate your right hand counterclockwise, until your thumb is on the outside of your body axis. The rotation is unencumbered in most people, and the rotation can continue all the way to the shoulder. Now try the same movement with your thumb extended such as in this grip. You might be surprised to discover that your rotational movement is significantly restricted! The same limitations occur in an opposite rotation with the elbow being forced to fold against the body to achieve the same rotational degree. The fact holds that with the thumb extended, motion is restricted and the thumb is more vulnerable.
I'm not claiming that this grip technique does not have its place, on small knives in close quarters combat. But for larger knives, it's probably not the best grip technique.