I think the parentheses are just for grouping of the second command. The "<" means feed the stdout of the second command into the stdin of the first.
So what's happening here is the output of curl (a script) is being fed directly into a bash shell and executed. I think it's basically an awkward way of piping a command in reverse.
using only the second "<" would result in bash being called with a argument like "/dev/fd/N", which is where the output of the subshell is coming from.
The first "<" redirects the output of this file descriptor into the stdin of bash.
So both are needed, but you could just do it with a pipe :/
So what's happening here is the output of curl (a script) is being fed directly into a bash shell and executed. I think it's basically an awkward way of piping a command in reverse.