Trihexphenidyl and Benztropine are antimuscarinics that can treat the resting tremor and rigidity of parkinsonism.

To expand on this, what we think happens with Parkinson's disease (and parkinsonianism) is an imbalance between dopamine and acetylcholine. It makes more sense if you look at this diagram, paying particular attention to the indirect pathway. Loss of dopaminergic (DA) neurons from the substantia nigra (SNc) results in constant activation of those ACh secreting neurons, which ultimately results in inhibition of thalamus from initiating movements. Therefore, using anticholinergics help with parkinsonianism secondary to haldol.