The inner unit muscles are tonic muscles that funcaton as stabilisers. They effectively stabilise the spine and sacroiliac joint at low levels of contraction with low susceptibility to fatigue.
Co-orination is critical for proper stabilisation. The ability of the inner unit muscles to contract prior to force production of phasic muscles (geared toward movement) is more important than their strength.
Reaserch shows that in people with no history of low back pain, the Transverse abdominis fires 30 milliseconds before arm movements and 110 milliseconds before leg movements.
Prior to movement of the body the inner unit is activated, contracting the Transverse abdominis and the Multifidi. Because the fibres of the Transverse abdominis are horizontally orientated, the umbilicus is drawn in toward the spine on contraction.
This explains why you would use your deep inner unit muscle's when lifting a heavy weight such as a deadlift. The unit must be activated and fire prior to lifting the weight.