Thus, as the boiling boundary moves up the tube, the total pressure drop falls, potentially increasing the flow in an unstable manner. Boiler tubes normally overcome this (which is effectively a 'negative resistance' regime) by incorporating a narrow orifice at the entry, to give a stabilising  pressure drop on entry.
Therefore, as the boiling boundary ascends along the tube, the total pressure decrease decreases, potentially causing an unstable increase in flow. Boiler tubes typically counteract this (which is essentially a 'negative resistance' regime) by incorporating a narrow opening at the entry, resulting in a stabilizing pressure drop on entry.