The corso was a Carnival tradition that promoted horse-driven and later, automobile-centric parades, where the cars were generally expensive, open and decorated. People were, of course, in costume, and threw confetti, serpentinas (streamers), and jets of lança-perfume* on the occupants of other vehicles. Once the cars had gone by, one could expect the start of the ranchos, which were a more middle-class affair.
The corso is of European origin, and it was popular in Brazil at the end of the 19th century and into the start of the 20th, as a way to reproduce the flower battles (Batailles des Fleurs) that characterized the most sophisticated Carnivals at the turn of the century, such as those in Nice.
By nature, it was also something exclusive to the elites, who owned cars or could pay to rent one during Carnival. By the mid-1930s and early 1940s, the corso died out, partly due to the fact that open-air cars were no longer made in Brazil and because the bigger trucks (before becoming trio-elétricos), carrying more people, were allowed to participate in the corsos.
[The drug is inhaled from a high-pressure tube, like whippets or “poppers.” It contains mostly ethyl chloride, plus a scent, and gives users a euphoric, short-lived rush.]