The indirect pathway does the opposite job. While the direct pathway promotes the chosen action, the indirect pathway suppresses competing alternatives. Both are needed for clean motor selection: amplify the right movement, silence the wrong ones.
Here is the indirect pathway in detail. Cortex excites the striatum. Striatal indirect-pathway neurons (which express D2 dopamine receptors) inhibit the external globus pallidus (GPe). GPe normally inhibits the subthalamic nucleus (STN). With GPe inhibited, STN is disinhibited and fires more. STN excites the internal globus pallidus (GPi). GPi inhibits the thalamus more strongly than usual. The result is more inhibition of thalamic output, which means less excitation back to cortex, which means competing motor programs are suppressed.
That sequence has more steps than the direct pathway, but the net effect is opposite. Direct = movement. Indirect = suppression of movement.
Dopamine modulates the indirect pathway through D2 receptors. Here is where it gets interesting. D2 receptors are inhibitory — they make indirect-pathway striatal neurons less likely to fire. So dopamine inhibits the indirect pathway. When the indirect pathway is inhibited, it suppresses less, which means more cortical motor activity gets through.
Notice the elegant symmetry. Dopamine activates D1 in the direct pathway (which promotes movement) and inhibits D2 in the indirect pathway (which removes suppression). Both effects produce more movement. Dopamine says go through both routes. This is the core mechanism behind why dopamine loss in Parkinson's produces bradykinesia and why dopamine excess (or D2 blockade) produces opposite syndromes.
When the substantia nigra dies, the indirect pathway loses D2-mediated inhibition. It becomes overactive, suppressing motor programs that should fire. Combined with the loss of D1-mediated direct-pathway activation, the result is the rigid, bradykinetic, tremor-laden phenotype of Parkinson's.
Conversely, when D2 receptors are blocked by an antipsychotic, the indirect pathway becomes hyperactive, producing the same motor suppression — bradykinesia, rigidity, sometimes acute dystonia. This is iatrogenic Parkinsonism, and it is one of the most common side effects of first-generation antipsychotics.