A cut from below.
The Unterhau (or variations of that name) can be found across several of the glosses. For example it is found in the section on the Krumphau from Lew at wiktenauer.com.
"When therewith one Wars [against] the Ox and also the Over- and the Under-hew, then drive thus..."
As well as the Zwerchau:
"Item, drive the Failer thus, when you come to the man with the pre-fencing, then hew the Under-hews from both sides. If you then come on him with an Under-hew from your right side, then shoot in the point therewith long in to the breast, so he must parry. Then spring quickly with the left foot on his right side, and do as if you will strike him thereto, but pull the hew and strike quickly around again to the left side. Or, if you come before the left side with the Under-hew on him, then shoot in the point yet long, and drive the driving as it stands before in the nearest description, etc."
And the Inverter:
"... with the left foot before, and hew a free Under-hew from the right side in accordance with each step forward, according to the left foot, and with the hew, so Invert and turn the long edge of the sword always above."
The exact description of the unterhau is not given as an explicit cut, however cross referencing glosses and other related sources such as messer texts and Talhoffer, suggest it is fundamentally the same as Meyer's version of the cut in the 16th century.
In Meyer's text this is written as the Unterhauw.