A Visceral Attack in Bloodborne is similar to a riposte from the Souls series. There are 2 main steps to initiate a visceral attack:
- Interrupt enemy's attack with a well-timed firearm shot (by clicking L2 with a firearm equipped); the enemy will then kneel down for a short period of time.
- Move up to the vulnerable enemy and tap R1; if you were quick enough, you should perform a visceral attack.
You can also perform a backstab-visceral attack by
- Moving behind an enemy and charging up an attack to stun them (holding down R2),
- Then moving up to them and tapping R1.
However, note that if you wait too long, the enemy will recover before you can perform a visceral attack. In addition, you cannot perform a visceral attack on certain enemies, although you can on certain bosses (e.g. Father Gascoigne).
You can read a detailed article on visceral attacks here.