The video shows a man transporting a fish from a bucket of nice fish-friendly water to a container filled with an unspecified substance (we know it's not liquid nitrogen, as the temperature is shown at -35 in the beginning, which is too warm for nitrogen to still be in liquid form, no matter the unit).
A subduction zone is formed at a convergent plate boundary when one or both of the tectonic plates is composed of oceanic crust. The denser plate, made of oceanic crust, is subducted underneath the less dense plate, which can be either continental or oceanic crust.
Where two tectonic plates converge, if one or both of the plates is oceanic lithosphere, a subduction zone will form. An oceanic plate will sink back into the mantle. Remember, oceanic plates are formed from mantle material at midocean ridges. Young oceanic lithosphere is hot and buoyant (low density) when it forms at a midocean ridge. But as it spreads away from the ridge and cools and contracts (becomse denser) it is able to sink into the hotter underlying mantle.