From their website:
This line represents the armies of the Chinantecs, a confederation of people from 20 towns in the Oaxacan highlands in what is now southern Mexico. They were fierce enemies of the Aztecs and so welcomed the Spanish as allies in their struggle to remain independent of Aztec conquest. Histories vary but some say that they had been conquered by the Aztecs just before the Spanish arrived in a series of bitter campaigns, others asserting that a few of the towns were still holding out when the Spanish overthrew the Aztecs.
Probably the most interesting aspect of their warfare was their employment of phalanxes of spearmen using enormous spears, reported, perhaps with some exaggeration, to be over 6m (20 feet) long. Even more unusual, these spears have razor sharp obsidian glass embedded along the lateral edge of the pole, running a considerable length down the top and bottom of the shaft. The soldiers who bore these spears also held a cloth shield called a pavesina by the Spanish, suggesting it was large and roughly rectangular, this shield apparently being of thick fabric so that when not in use it could be rolled up and carried.