Please describe to me, in detail, how this works.
Because every time this comes up everyone claims it's the easiest thing in the world, but if you try and drill into it what they end up actually wanting is generally "pay what you want" cloud services.
There are a _ton_ of resources on AWS that accrue on-going costs with no way to turn them off. A "hard circuit breaker" that brings your newly accruing charges to zero needs to not just shut down your EC2 instances, but delete your EBS volumes, empty your S3 buckets, delete your encryption keys, delete your DNS zones, stop all your DB instances and delete all snapshots and backups, etc, etc.
The only people I see using a feature like this are some individuals doing some basic proof-of-concept work and... a bunch of people that are going to turn it on not understanding the implications and then when they get a burst of traffic that wipes out their AWS account they're going to publish angry blog posts about how AWS killed their startup.
If, like most people, you don't want literally everything to disappear the first time your site gets an unexpected traffic spike, you can already do this by setting up a response tailored to your workload--run a lambda in response to billing alerts that shuts down VMs, or stops your RDS instance but leaves the storage, etc.