It is based on an effect called the simple spending multiplier. The simple spending multiplier is a number showing the multiple by which real GDP changes for a given change in spending. We know that we can think of GDP in two ways. It is the value of production and it is the income from production. You know production yields income.
Income generates spending.
When production increases, income increases. Increase in income dictates an increase in spending. They will buy cars, haircuts, paper towels, etc. These suppliers now have more income to consume or save. At some point the new rounds of income and spending become so small, that they disappear and the process stops.