It's doing if(cond, effect1, effect2) where effect1 and effect2 are functions, and only evaluating the matching effect function. But everything is functions.
technically effect1 and effect2 are so called "blocks of code" in rebol/rye/red/...
Everything being a function is trying to say that every "active word" (a word that does something ... print, first, if, fn, context, extends, ...) is just a function.
Everything being a function is trying to say that every "active word" (a word that does something ... print, first, if, fn, context, extends, ...) is just a function.