Hold Down in the instant after using a flask and before your character makes the sound that confirms he or she has been healed. If done correctly, the flask may be re-used to fully heal your character.
Extra Life From Healing Flasks
This code gives you more life out of a flask. When you're playing as Carnby or the girl, drink the flask and watch carefully. When your character's arm goes down before he/she says "Uuhhmm!", hold DOWN for one second. If your timing is right you'll re-drink the flask and heal again. Repeat as necessary until you're fully healed.