http://www.myarmoury.com/feature_spot_rondel.html
The above article is an interesting read.
In regard to your specific question, I can't comment on Lignitzer. Instead, I took a glance at my copy of Talhoffer (Medieval Combat-- Mark Rector). The dagger plates possibly indicate the thumb toward the pommel in the "ice pick grip" only twice in Plate 180. By immediate contrast, there are many images of the same "downward" grip with the thumb clearly wrapped over the fingers in Plates 170, 171,173, 175 just to name a few. In regards to a grip (hammer grip) where the blade would face up when held naturally at arm's length, Talhoffer shows the thumb toward the upper "crossguard" disk in Plate 172, 175, 180, and 181.
This sampling would lead me to respectfully disagree with Mr. Bertucci, and I would gladly welcome any evidenced refutation. Until then, I would suppose the thrust you mentioned would be held in the hammer grip.
I am also unsure as to the advantage of using or negelcting the thumb in either grip by I haven't studied dagger yet.
Former member Jason Vail has researched dagger extensively and his work, including his book, may be of some value to you. Best of luck.
Doug Marnick
NYC