This was a style done at the turn of the century and my great great grandfather actually wrote this style. After showing the examples I have of his with Michael Sull, they were done with a straight holder. It was typically referred to as flourished script in the US, and very few penman did it due to it's complexity.
A straight holder is held in the traditional offhand flourishing style, where the point of the nib is towards your body and it is written from left to right. As a result the upstrokes (thicks) are pulled away from your body and the downstrokes (thins) pushed towards your body.
It can be accomplished the way Erica described, but the old timers (and those at the time of printing of the Ames guide) did it in the way I described.