The 35% magic penetration from Void Staff becomes more effective the more magic resist the enemy target has on them. Ranging from 30 magic resist to 200 magic resist, the magic penetration increases damage from 8% to 30%.
Trivia
This item has an AD counterpart which is the Last Whisper.
Champions Abilities
Percentage Magic Resist penetration stacks multiplicatively with others champions percentage Magic Resist penetration abilities.
Void Staff to be gold efficient.
Devastating Strikes yields 38.9% magic penetration, rather than 41%, because percent magic penetration stacks multiplicatively.
Last Whisper.
Blasting Wand + 
Amplifying Tome + 1000g.
