How does pressure help patients who are suffering uncontrollable hemorrhaging..?

Well, it obstructs the blood flow to lessen bleeding.

Hi. Local pressure will stop local bleeding by squeezing off the arteries and capillaries.. Never heard of pressure being used internally, but it may have the same action.

Are you refering to pressure on the wound? or are you refering to increasing blood pressure.  Obviously increasing blood pressure would maintain cerebral and cardiac blood flow but you would hemmorrhage must faster.  For wounds, putting pressure on the wound would hopefully stem the bleeding.

Its just a way of closing off the blood vessels that are perfusing blood out of the patient's body.  The more pressure, the higher the resistance in the blood vessels in that area, and so the blood will hopefully flow somewhere else where there is less resistance (and intact vessels).

The body also has mechanisms that come into play during hemorrhage to raise internal mean arterial blood pressure to maintain flow to the brain and heart.  It does this chiefly by constricting the smooth muscle around vessels in uneeded areas, like the gut and skin.

Pressure slows the flow of blood allowing the body to either clot off the wound or a MD to repair it .  The body is capeable of cloting it's self off without pressure for most small wounds but wounds to an artery or major vein ( where the pressure is greater ) require some intervention.  In the medical community people joke that "all bleeding stops , eventually" (gallows humor) .
But seriously,
Any bleeding (of the fairly small , superficial variety)  that won't stop after 60 minutes of constant pressure (no peaking ) , that gushes or squirts with each heart beat , or is obviously life threatening needs to go to ER .