The neck role supports the curve of the cervical spine. Without that support, the neck moves which are taught in Trager® classes become constrained by any cervical vertebrae that are rotated or out of alignment. The neck role really feels good to the client and as a result, any kind of movement of the head, neck or upper torso allows the neck to move freely, in spite of any protective constraints by the cervical muscles.
Over the years as a Trager® practitioner, I learned to use my hands to support the cervical curve and then gently tap on tight cervical muscles to bring the client’s awareness there, releasing the strain. 

The cervical roll allows immediate movement of the head and neck when you move any part of the client’s body.  And that movement not only feels good to the client, but also often it allows the vertebrae to go back into alignment. This is especially helpful at the juncture between the cervical and thoracic vertebrae. Using your hands to move the ribcage side-to-side creates comfortable movement for the whole torso.

The bolsters underneath the knees create a cantilever effect for the low back and pelvis due to the suspended weight of the lower legs and since the lumbar spine is flattened any side-to-side movement of the pelvis is very freeing to the lumbar spine, especially the quadratus lumborums in the back and the psoas in the front. Any movement that is done with lte legs in this position redounds to the whole body. Once again because of the suspension effects of the bolstering, even very gentle movements redound to the whole body.