I am the ninth remedy master of dagger and I no longer hold a dagger. And this grip that I do against an attack from below is the same that the fourth remedy master of dagger makes against an attack from above, except I do it below. But my plays are not the same as his. The grip is worthy in armour and without, and from it I can make very strong plays, especially those that follow me. In armour or unarmoured, they are not doubted.
As Fiore points out, the cover of the 9th remedy master and the cover of the 4th remedy master are essentially the same. They are just applied to a different set of circumstances. The 4th master defends against a fendente strike and tends to direct the opponents wrist. As the 9th master, you defend against a sottano attack, and more direct your opponents elbow. The mechanics of this means that although the grip is the same, the plays which follow from each different master are quite different to each other.
In both cases, the cover is the same you would use to grip a sword. With your left hand, take your opponents wrist. Grip tightly with the thumb and bottom two fingers. Use the top two fingers to provide direction and control. Your right hand grips half way up the forearm in a similar manner.
From here, you have lots of control over your opponents arm. You can easily manipulate their balance, and transition onto the plays.