In most scenarios and whenever possible, the best self-defense tactic is non-engagement by evading. Otherwise it is to engage the assailant(s) through direct physical interaction – combat.
However, there are certain exacting scenarios where disengagement could be effective with ‘self-inflicted defense’ when situated in a preengaged position.[ Further Intel: The 4 Types of Self-Defense Engagement Responses ]
The objective is to deter the assailant(s) from engaging you by using blood. Produced by your own by non-critical self-infliction or otherwise fresh wound.
Rather by showcasing that you’re already injured and pose no threat (for active combat scenarios) or to be perceived as too “dirty” covered in blood to even be touched (for muggings, street assaults).
This is a process in theatrics and is only to be used as an only / last resort as per special circumstance.