The power stored using the move Stockpile is released at once in an attack. The more power is stored, the greater the damage.
In-Depth Effect:
This move has four tiers of damage; 3 are stockpiled, 1 isn't. Unfortunately the one that isn't is that the attack doesn't do anything if you haven't stockpiled so you have to make sure to Stockpile first. If you use Spit Up after 1 Stockpile, the Base Power will be 100 If you use Spit Up after 2 Stockpiles, the Base Power will be 200 If you use Spit Up after 3 Stockpiles, the Base Power will be 300