What transmission method allows for signals to flow both directions but not at the same time?

The transmission method that allows signals to flow both directions, but not simultaneously, is half duplex. This mode of communication enables information to be sent and received in alternate directions. It's commonly utilized in situations where devices must take turns transmitting data, such as in walkie-talkies, where one person speaks while the other listens, but they cannot communicate together at the same time.

This characteristic of half duplex highlights its difference from full duplex, which allows for simultaneous two-way communication. In contrast, simplex communication only permits data to flow in one direction, with no ability for feedback or return paths. The term "bi-directional" may imply simultaneous communication, which does not match the definition of half duplex, where back-and-forth communication occurs, but only one direction at a time. Thus, half duplex is specifically defined by its capacity for two-way communication without overlap, making it the correct choice.