I play with the arms crossed to make those remedies that have come before. And if we were both armoured, I could not make a better cover. A stronger remedy than me does not bear a crown, for I can make plays to the right and left. Also I can cross both below and above.
The 2nd Master of Dagger fights from Posta Tutta Posta di Ferro, Incrosada e Dopia. This is a posta which notes the value of armour and its inability to make long covers. The mechanics of crossing the arms means you simply cannot reach out very far, making for extremly close play with very small margins of error. It is the close proximity to a dagger point which leads to the advice of only doing this with proper protective equipment.
At first glance, it seems as though you are blocking the attack in the cross of your forearms, but there are a number of subtleties happening here. As the dagger approaches, you are actually attacking the players wrist with your forearm. You will need to extend your arms far enough to keep the dagger clear, but not so far as to weaken your own structure.
Drive your attack with the left hip, and initially lead with the thumbs up. Immediately on contact, roll your hands inward so that you strike the players wrist with the ulnar edge of your left forearm. This roll will serve to protect your own arm and increase the force of impact. Your attack will stop the dagger while simltaneously jarring and hurting the players arm.
This is easier to do if you can catch the incoming attack before it has built up too much momentum. Get in close to your opponent using your left foot to close the distance.
From this position, you can also perform all the plays of the 1st Master.