offen halten

The core theme of this root was

(up) from under, going above

So basically a motion from down below to up above.
Some English sources (etymonline.com) mention two separate roots for similar themes here, but according to the German source I am using (DWDS.de), this root is the origin of both, syllables that are about “under” and syllables that are about “over”.

• hypo- (“under”)
• sub- (“under, below”)
• super- (“above”)
• sur- (“above”)
• hyper- (“over”)
• up- (“upward”)

There are countless verbs and words with these prefixes, of course, including some that are not immediately obvious as prefix words:

• succeed (“come after”)
• supply (“fill from below”)
• suffice (“put under”)
• suffer (“to bear a weight, be under pressure”)
• source, surge (“spring from below”)
• substance (“what something stands on”)

Besides those, the root is also the origin of words like up, often and open.
The idea of open comes from the notion of being out in the open, being exposed, which you’re also are when you go upward.