Leech Seed is a move well-known for absorbing 1/16 of the target's HP and healing the user of the move by the same amount. However, you can actually boost this damage through the unintended effects of Toxic. When you use Toxic to Badly Poison a foe, and then use Leech Seed on them, Leech Seed's damage will also increment: thus, you regain 1/16, then 1/8, then 3/16, then 1/4, and so on, of your HP. Toxic will also be taking effect in this time, dealing the same amount of damage. In other words, you not only regain tons of HP, but also double the damage of Toxic, from one point of view.