Crowding In. An increase in government spending causes an increase in private investment (maybe the government invests in the infrastructure which encourages private firms to invest).
An expansionary fiscal policy may cause crowding-in, that is, it may cause private investment to increase. An increase in government spending will increase income and consumption so firms will increase investment to sell more and make more profit. Maybe an increase in government spending on the infrastructure decreases firms’ costs and incentivizes firms to increase investment to produce more to make more profit. AD shifts right because government spending rises and AD shifts right again because investment rises.