There are many common vocal myths concerning breathing.
A primary fact is to realize that we breathe with our lungs which are in our rib cage. We cannot breathe into our bellies nor into our pelvis nor into our backs. If one were to consciously attempt to breathe into the pelvic area, then what would actually occur is the creation of unnecessary muscular tension.
Yes, there are some voluntary muscles that one can flex within the breathing musculature. However, there are involuntary muscles as well, which means that the most natural functioning of the respiratory system is involuntary. Therefore, as singers who desire to function according to the laws of nature, then we respect and work with nature rather than imposing ourselves upon nature through overt mechanistic means.
All that said, when the breath is automatic and the lungs fill up, there can be a feeling of expansion in the pelvic region as well as the back. One does not do this, but can rather pay attention after the fact that it has happened.
What mostly one will feel is that they have “caught the air up” and sing with the air that is at the top of the lungs right beneath the larynx. This feel of having the air caught up just means that the lungs are fully inflated with expansion in other areas as one develops; allowing this to be a process that is based on the principle of automaticity. Let go and let your body do the work!