How could the 4/5 devaluation of Franc Poincaré in 1928 end the financial crisis France was going through?

Essentially, rob the poor to pay the rich

French franc, like any major currency at that time, was backed by gold. Franc Germinal was valued at 0.3225 g of gold. This was instituted in 1803 and did not change until 1928. Theoretically, your savings in francs were as good as gold, and many poor and middle income citizens held their life savings in government issued bank notes or coins. Of course, in reality Banque de France issued more currency then they had gold, but this system worked because only small amount of franc holders would chose to exchange their currency for real precious metal .

However, when WW1 started France simply did not have enough money to finance the war. Already during the war they started issuing francs without gold backing (and prices of everything, especially food, skyrocketed) but they needed loans also. Bonds for these loans were formally issued in Franc Germinals , but creditors, like famous Rothschilds, knew very well that even if France wins it would be very difficult to pay both principal and interest on these loans. However , they counted on something else.

After the war, France was saddled with debt, quantity of money in circulation was enormous, and to preserve international standing franc continued to be valued at same ratio towards the gold. Despite hefty German war reparations, French economy could not sustain such ratio, especially since creditors could at any time demand to be paid in gold (or some other currency backed by gold) .

Since French gold reserves were getting depleted, Poincaré simply devaluated franc to only 20% of its previous value, namely 0.0655 g of gold. Seemingly, this hit large holders of French debt hardest, but in reality they already charged vast sums of money (i.e. gold) on interests, and had protection clauses in their contracts so their holdings were not wiped out. Essentially, Poincaré put repayment to creditors as No 1 priority.

Hardest hit were common citizens, especially those on fixed income. If someone had 100 franc income (wage or pension) or saving, after the devaluation he would have only 20% of the value . French wages suddenly became low, and French products cheaper in international market, at the expense of working class.

Finally, idea that Poincaré somehow solved economic crisis is blatantly wrong. In fact, it could be argued that Poincaré created moral decline that led to speedy French collapse in 1940. For average French citizen, relative fiscal stability did not mean much in face of overwhelming poverty, Great Depression and general feeling that poor paid the price of WW1, both in blood and treasure. Already in 1934 there was political crisis that heralded deep divisions in French society, and general distrust in state institutions. As WW2 draw nearer, it was becoming clearer that French were no longer unified body ready to confront Germans once more on battlefield.

Clarification of some basic economic facts: War as an activity from economic standpoint is wasteful - valuable resources are either destroyed by enemy, or spent destroying that enemy instead of producing something valuable. During the war French economy contracted, as were economies of other belligerents. French government also increased money supply, therefore value of franc was certainly lower (many more francs chasing lower amount of goods). Banks and big investor perfectly understood this (while general population, same as today, did not). Banks understood they were loosing wealth, but needed a way to limit the loses, and grab larger percent of economy. They also understood that at some point economy will start growing again, and those who control significant percent of it will reap most profits. That is why they started buying war bonds, although they knew perfectly well that they are not going to be payed back in gold as promised. Even during the devaluation bond holders got preferential treatment compared to ordinary citizens. They were not paid back in full, but they didn't expect that to happen. Instead, they got tighter control of economy, by sucking in enormous part of French franc monetary mass.