What's In a Vessel?

DS's trigger for eliminating does not appear to be the "pss-pss" sound but instead is when the feeling or act of sitting on his plastic potty chair. DS gets stagefright when I put him on the big toilet seat or hold him over the toilet. EC experts say that the vessel shouldn't matter, so that you are free from having to tote the potty training chair around with you in the car. But for now DS is only comfortable doing the duty on his own toilet, his green portable potty chair.