You amplify the targets will to live to such a degree that the cannot die through sheer willpower alone.

When the targets HP is reduced to 0 while the effect is active, they will instead go into the negatives. Negative HP translates to a modifier on your death saving throws. when the effect ends. For example if you had -12 HP when the effect ended, you would get a -12 modifier on your saving throw. When the modifier is higher than 20, it affects your next saving throw, i.e. -24 HP means one lost saving throw and a -4 on your next throw.

Dropping to -50 HP is an automatic death when the spell ends.