He's not actually releasing it, but he is charging it.
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Containers (コンテナ Kontena) are cubic-shaped boxes, which are generally seen set around of stages...
Sonic Adventure 2 (Wooden Container) Sonic Adventure 2 (Steel Container)
The Hold Smash (ホールドスマッシュ Hōrudosumasshu) is a technique used by Silver the Hedgehog in Sonic...