"down" used as a verb?

According to the OED it is a US sense, which I (a British person) had never before encountered.

It is sense 2b, of verb2 of "down", meaning "to die down". Only one example is given.

2b. To die down. U.S.

1924 W. M. Raine Troubled Waters xvii. 180 The rumour would not down that one of the prisoners had turned State's evidence.

verb1 - down has to do with the application of "down", meaning feathers.

verb2 - down relates to various verbal senses to do with a downward direction.

Having worked in computers for many years, the term "down the interface", which means to put into an offline state making unavailable for use but not necessarily to power the entire machine down.

For example, when performing an upgrade it is sometimes required to down the interface, which means to take the network interface from being online and available to being offline and unavailable, preventing connections to the machine from being established until the software upgrade is complete. In this example, if you were to look at the interface, it would very literally display "down" as a status.

"We need to down the interface on Saturday. Please let everyone know."

You can also use it to indicate shutting something off, such as down the server or down the lights. In any case, this would be a common usage of down as a verb as opposed to any dictionary defined usage.

https://www.merriam-webster.com/dictionary/enter%20common%20usage

In your American Heritage example, the use of the term down as a verb means that it persists. I cannot find any usages of down as a verb in any modern context except as shorthand for a more complete phrase such as "turn down the volume", where if you said "Hey, down the volume" people might know what you mean, but it would not be a proper usage.

There is a phrase "down the hatch" but the meaning is not the same, even if the structure is. There is no hatch to put in a down position, rather it refers to taking a drink until it is finished.

https://en.oxforddictionaries.com/definition/us/down_the_hatch

English is not confusing at all, said no one ever.