This video was produced by Sifu Scott Rodell from the Great River Taoist Center, demonstrating his interpretation of the Flying Dagger Stance from the Dandao Chinese Long Saber manual.
As you can observe in his video, the dagger is held in a normal grip with the blade pointing upwards. But in the actual manual, the dagger is drawn being held with a reverse grip, blade pointing downwards.
Personally, I have 2 theories about this. Both have their own pros & cons.
Firstly, if the dagger is held in a normal grip, blade pointing upwards, the swordsman can make a more accurate throw. However, there are 2 cons:
The dagger can't be concealed as easily, and the enemy may be able to see that you're up to no good with the dagger.
Because the dagger is sashed with the blade pointing downwards, retrieving it with a normal grip may be a bit slowly and awkward.
Second theory, because the dagger is already sashed with the blade pointed downwards, retrieving it with a reversed grip is smoother process. Plus, hiding the dagger from the enemy's view is also easier.
The disadvantage is that, the swordsman cannot make an accurate throw. The blade of the dagger may not even hit the enemy too! However, if you are merely throwing it to distract the enemy, so as to create an opportunity for you to rush in and attack, the inaccuracy in your throw is probably not an issue.
Ancient Chinese Martial Arts Manuals