This theory is also known as hypodermic needle theory, magic bullet theory and, powerful effect theory. According to the prior scholars of the communication, the mass communication medium is that much effected that they can easily change people ideas according to their own. Even they also have that much power to change the voting decision of people. This is a sender-receiver centric model. According to this model, the sender sends a message and receiver accept it as it is.
One step flow theory direct hits the people minds and change their ideas, thoughts regarding any particular issue. The receiver is passive here and accepts everything that comes through mass media as it is without any reaction.
This theory was basically originated in the year of 1930, in the context of Behaviorism. It was based on the early observation effect of mass media on people. This can also be understood by graphically
This theory assumes that people get influenced by media messages (bullet) fired from the media gun into the viewers head. Similarly, hypodermic needle theory assumes that the media injects the ideas into viewers head and audience accept it passively. The audience can not escape from the effect of the media. This passive audience direct influenced by the media.
That time lack of communication tools and lack of public understanding it was easy for the mass medium to influence anyone effectively and that could be also the reason for people vulnerability.