Gripping the Dussack
From Scholar Victoria
Like the longsword, there are several ways to grip the dussack.
The Sabre Grip
Many of the illustrations show a sabre style grip for the dussack. In this grip the weapon is held comfortably in the hand with the fingers clasping the haft. The thumb, however lays down the spine of the haft either directly on it (at the 12 o'clock position, or just on the corner of the weapon (at around the 10 to 11 o'clock position). This is roughly the same as the sabre grips seen in Hutton or, in the second case, Waite.
This position is used for cuts and thrusts (it's particularly effective for these) as well as for crooked cuts (krumphauw).
The image below shows the sabre grip on the dussack.