code: public function getSlave($fallbackToMaster = true)
      {
          if (!$this->enableSlaves) {
              return $fallbackToMaster ? $this : null;
          }
  
          if ($this->_slave === false) {
              $this->_slave = $this->openFromPool($this->slaves, $this->slaveConfig);
          }
  
          return $this->_slave === null && $fallbackToMaster ? $this : $this->_slave;
      }
docs: Returns the currently active slave connection.
  If this method is called for the first time, it will try to open a slave connection when [[enableSlaves]] is true.
  @param bool $fallbackToMaster whether to return a master connection in case there is no slave connection available.
  @return Connection the currently active slave connection. `null` is returned if there is no slave available and
  `$fallbackToMaster` is false.