In healthy individuals, the pelvic floor and core abdominals work as a team (along with other muscles including the multifidus and respiratory diaphragm) to help provide support and structure for the spine and pelvis. Again, to be clear, the core abdominals and the pelvic floor are different muscles, but are supposed to be wired to fire together.
For example, in a healthy individual, when the pelvic floor is engaged, the core abdominals co-contract subconsciously with the pelvic floor. In the office, I check for this using Real-Time Ultrasound Imaging. It occurs the other way around too, but with a slight difference – when the “team” is led with a core abdominal contraction, the pelvic floor co-contracts subconsciously with the core abdominals, but at a smaller percentage of the pelvic floor’s potential for contraction. This is an important point, because if there are incontinence or prolapse concerns, leading the “team” with a core abdominal contraction may not result in a pelvic floor contraction that will be enough to provide continence and organ support.
All of this is assuming that the pelvic floor and the core abdominals are functioning and firing optimally, together as a team. It is not enough to rely on and assume that these co-contractions always occurring. The structure and function of this team of muscles can be altered or lost in the presence of pain, injury, surgery, or with physiological changes that occur in pregnancy and delivery.