1-wire differentiate read from write
The master is the one who decides the timing on the bus. Therefore to start clocking out data bits from slave it must send a short low pulse, and depending on which data bit the slave wants to send it will either do nothing or pull the line down so it looks like a long pulse. The master then determines the bit if the pulse was long or short.

But before the slave sends out anything, the master needs to send out a command packet (e.g. please send me temperature data) which puts the slave into transmitting a response (e.g. here is temperature data) back.