What happens when we are not at equilibrium?

What happens when we are not at equilibrium i.e. at Y1 or Y2?

When the economy is at Y1, planned expenditure (at B) is greater than output. Firms are not producing enough and this results in a rundown of inventories. The fall in inventories will induce firms to increase their output. This causes the economy to move toward the equilibrium at Yo.

At Y2, however, planned expenditure (at C) is less than output. Firms are selling less than what they are producing. The increase in inventories induces firms to cut back on production. As output and employment falls the economy moves back to the equilibrium level of output, Yo.