What bash < < syntax mean?
Solution 1:
It's process substitution. It feeds the output of the command into a FIFO that can be read from like a normal file.
Solution 2:
It means "run the command inside the brackets, and return a filename that represents the standard output of that command here".
So, that translates to two commands:
curl ... > something
bash -s stable < something
...where "something" is the magic. (Typically, /dev/fd/... or a pipe.)