There are two effects, one of which will apply depending on the situation the user is in:
The first effect applies to any PvP situation. As the description states, it steals all the adrenaline from your target, leaving them with no adrenaline while you gain adrenaline equal to the amount they had.
The second effect can be partially misleading, as there are several monsters that utilise adrenaline, such as Cadarn rangers and magi. When used, it doubles most sources of adrenaline gain (such as Telos' green anima stream and using abilities, but does not affect adrenaline potions). The adrenaline doubling effect will apply outside in a PvE situation, excluding Nex's praesuls, where the first effect is applied instead.
With the second effect of Natural Instinct, one can gain 150-200% adrenaline (this does not including eating, which drops adrenaline) with the 30 seconds that Natural Instinct provides. To make it more efficient, one can use an ultimate skill such as Sunshine to further increase damage output while gaining a lot of adrenaline to make use of threshold abilities.