Uber Technologies Inc. has begun locking New York City drivers out of its app during periods of low demand in an attempt to fight a minimum wage rule, and Lyft Inc. is threatening to do the same. As a result, some drivers say their wages have fallen by as much as 50%.At the heart of the move, say the two companies, is a six-year-old pay rule in New York that, among other things, requires firms like Uber and Lyft to pay drivers for the idle time they rack up between rides. The lockouts, which beg...