Krachai has numerous health benefits, many related to immunity against infections. It has a long history as food ingredient, which means that humans are well adapted to its ingestion.
Fingerroot's sexual applications is what is of interest to us here. Fingerroot is a potent phosphodiesterase inhibitor, albeit of a low affinity. Sildenafil, the active ingredient in Viagra, is also a phosphodiesterase inhibitor.
Phosphodiesterase inhibitors cause and prolong erections, this is well-known.