connection has separate transmit
lines, which means that the hardware can send data and receive data at the same time ("full duplex"). However, the majority of the protocols are half duplex: also called "master-slave" or "query-reply". What the splitter cable does is to re-route the transmit
lines of both devices to the read
line for the monitoring PC. This will only work correctly for half duplex protocols; when both devices that are involved in the serial connection send at the same, the signals will collide on the split-off receive line.
The two 4k7 resistors cause some cross-talk between the transmit lines of the two communicating devices