What is the difference between flexible and rigid accelerator in economics?

In economics, a flexible accelerator refers to a model where investment levels adjust in response to changes in output or demand, allowing firms to quickly adapt to economic conditions. In contrast, a rigid accelerator implies a slower adjustment process, where investment decisions are based on fixed relationships or past levels of capital, leading to less responsiveness to current market conditions. This distinction affects how quickly economies can respond to shocks and how capital is allocated over time.