Through what Keynes calls "public works." Our nation provides a number of things to the people that reside here. It provides military protection, infrastructure, opportunities for companies to do business, etc. In exchange for all these things that we provide as a nation, individuals and companies are required to pay taxes. That tax money can be reinvested in public works that introduce money back into the demand side.
If the extra money comes from printing more money, then yes. While hyperinflation is disasterous, as Germany discovered not to long ago, a little bit of inflation is a good thing. Most economists and central banks aim for inflation around 2%. Printing more currency is not the only source of more demand-side money though.
Propensities for savings and consumption both increase with income, but increases in propensity for consumption will diminish as income increases whereas increases in propensity for saving will escalate. For a given income Y and a propensity for consumption C(Y), the propensitiy for saving S(Y) can be determined by S(Y) = Y-C(Y).
View attachment 67301523
As income increases, the portion of savings capital being removed from circulation increases while the amount being recirculated through consumption diminishes. Because of this, reduced taxes on higher income has the net result of less capital in circulation, compared with using increased taxes from higher incomes to pay for public works that stimulate the economy from the demand side.