When a yakuza sees a hearse (霊柩車/reikyusha) coming towards him in traffic, he’ll often put down the cell-phone which he has semi-permanently attached to his ear, and hides his thumb in his fist to avoid bad luck. (親指を隠す). The thumb, called the oyayubi 親指 in Japanese, means “parent finger.” It represents to the yakuza, the oyabun (father-figure) who is their boss in the organization. The Godfather so to speak.
The idea is that by hiding the thumb from the hearse they are protecting their boss and themselves from death or misfortune.
Conversely, if you’re a tattooed man, finding yourself in traffic riding behind a hearse is considered lucky and it means that you are going to have especially good luck gambling.
No one is really sure what it means to crash into a hearse but this is probably not a good thing.