I think it's this aspect that gets somewhat underplayed - and not just Poincare/Viviani but Maurice Paléologue, the French ambassador in St. Petersburg as well. As much as Germany gave AH a blank check, Paleologue came close to giving Russia a blank check from France. And in both cases, Russia and Austria-Hungary felt that they needed to cash it urgently, or else risk losing it and embarrassing themselves in front of their allies.
EDIT: And to add - the logic of deterrence was screwed up in both cases, because it was more private than it should have been. AH felt confident of German backing, and Russia felt confident of French backing, and therefore, they were both more aggressive than they would be otherwise—they may have even taken it as encouragement to be so aggressive. However, these positions were poorly communicated to their rivals, so that Germany proceeded as though French intervention was NOT certain, and Russia acted as though Germany might be convinced to stand down.
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