Ctrl-O - why subshell only with bash, tcsh, zsh?

[Alexander Kriegisch]

I hope you do not mind me pushing this:

Alexander Kriegisch:
> MC FAQ:
>> 6.6 When I use Ctrl-O I don't get a subshell.  How do I fix this?
>>
>> Only bash, tcsh and zsh can be used as subshell.  Use one of those 
>> shells as your default shell, and it will be used as subshell in GNU 
>> Midnight Commander.
> 
> What is the reason for that? I use BusyBox ash on an embedded platform
> (DSL/LAN/WLAN router) and would like to have a Ctrl-O subshell. What
> would be technically required from a candidate shell in order to make it
> work?