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I'd like to know how they cut/released the tether, and if there's any fallback in case 1st release attempt fails. The entire mission is fucked if the tether release failed, or if the sky crane tries to fly away while still tethered.

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They use explosive bolts that attach the tether to the deck of the rover. They're almost 100% fool proof. Here's a Wikipedia entry, they're called pyrotechnic fasteners and NASA uses a lot of them.

https://en.wikipedia.org/wiki/Pyrotechnic_fastener

But the image of the cables show what looks to actually be a literal cut cable. Not just a connector being disconnected but a clean slice.

From today's Reddit AMA with the NASA team, they use pyrotechnics to set off a guillotine like device to cut the cables. Possibly something like https://psemc.com/products/pyrotechnic-cable-cutter/

Source: https://www.reddit.com/r/IAmA/comments/lpzbzo/were_scientist...

I don't know how they achieved it specifically, but there is a moment in the broadcast[0] they did where they show the umbilical on the rover and mention that it was cut. I'm curious as well how they did it!

[0] https://youtu.be/gYQwuYZbA6o?t=1898

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