According to a pay-walled article in The Information [theinformation.com],
Thanks to a secret software-based effort within Uber called “Hell,” Uber could track how many Lyft drivers were available for new rides and where they were[.] [...] “Hell” showed Uber employees which of the tracked drivers were driving for both Lyft and Uber, helping Uber figure out how to lure those drivers away from its rival.
additional coverage: