The second half of the alchemist tree consist entirely of talents that depend on Geralt being poisoned. What exactly does it mean in the context and is there any way to induce poisoning at will?
Poisoned is a misleading term here - what really triggers this is toxicity, which is increased the more potions you drink at one time. The toxicity of each potion varies, but you can see it when you look at a potion's recipe or when you add it to your active potions during meditation. The bar that fills as you add potions is your toxicity meter.
Drinking a combination of potions that pushes your toxicity meter near the maximum will put you into a state where these bonuses are applied.
We also covered potion toxicity in this question: