IspellUsePrefixes defines if search.cgi should use prefix rules to generate word forms when doing Ispell-based stemming.
By default, mnoGoSearch does not use prefix rules, and uses only suffix rules for word form generation. Prefix rules usually change a word meaning inside out:
normal -> abnormal natural -> unnatural
When IspellUsePrefixes is set to yes, search.cgi starts using prefix rules to generate word forms.