There is a similar question here:
Control-r reverse-i-search in bash: how do you “reset” the search in Cygwin?
Found another similar question on Super User:
(reverse-i-search) in Bash
Apparently, both mention Ctrl+s, which may do the trick.
Hope that helps. I myself am trying to find a piece of code that does the reverse-i-search in order to check how it has been implemented exactly.