In supine would not necessarily aggrevate the problem as discs unloaded.

Flexing allows more room for the disc to protrude posteriorly, which again unloaded may not aggrevate symptoms....key though is what happens when they stand after this...tend to get 'nut cracker' type effect and protrusion squeezed and causes pain when standing.

Patients with posteriorly prutruding discs often sit slumped, flexed, to one side etc..and get relief....it is often hard to pursuade them that this is not necessarily the best thing for them!