This effect is obtained through the subtractive synthesis technique. This course doesn't treat the topic of sound synthesis and its implications, although it would be compelling to do so. For our needs it will suffice to say that subtractive synthesis consists in the removal, using appropriate filters, of certain frequencies which made up the original sound. Such filters are controlled by parameters which depend on the input signal, called VCF (Voltage Controlled Filter).
The following are a vocal sound and its manipulation through the application of a vocoder effect: