How can Hyouma see and use Gennosuke's technique when he permits to do so?
Naturally, the answer contains spoilers.
Because Hyouma is actually the one who taught Gennosuke his eye technique. Gennosuke inherited the Dojutsu skill from his mother, and as his uncle, Hyouma also possesses the same Dojutsu skill. However, Hyouma's case is special as

In the anime,

Hyouma's Dojutsu is constantly activated, forcing him to keep his eyes shut at all times to avoid killing the wrong person at the wrong time.


In the original novel and manga,

[Hyouma] was born with light-sensitive eyes, [and therefore] is only able to open them at night or other low-light conditions.



Reference: Basilisk Wiki.