> Do you know how this is actually being done on process level?

You spawn a new haproxy instance yourself, and pass it the PID of the process 
you want to replace:

      haproxy -f haproxy.cfg -sf $(pidof haproxy)

The new instance uses SO_REUSEPORT to attach to the same port, and then sends a 
FINISH signal to the given PID to gracefully shut down the running instance. 

Thanks,
Remko

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