Yes, the person would yell once you fell, but only if you fell. "If" and "Only if" used in the same way means the same thing, except that "only if" is more forceful, more compelling. "If and only if" is the most obligatory of the three, in which the action has been distinguished and emphasised, "If, and only if " It's the most forceful of the three It only ever seemed like a perfectly natural usage to me (I'm sure I've been using it freely for half a century or more, here in the UK).

Understanding the Context

So I was a bit surprised to find that my NGram suggests it's only only recently gained significant traction. Which is grammatically correct? I can only do so much in this time. or I can do only so much in this time.

Key Insights

grammaticality - Correct position of "only" - English Language & Usage ... meaning - "If" vs "Only if" vs "If and only if" - English Language ... P2. only but (also but only): (a) only, merely; (b) except only. Now poetic.

Final Thoughts

Source: Oxford English Dictionary (login required) Below are some only but examples from the Corpus of Contemporary American English. Swap in only or nothing but for only but to see: Ultimately, there is only but one choice for you, no? To consume the entire pint. TV ... For Only in the upper class, the only restricts in the inclusion (in, as opposed to out). But no matter which you restrict, there are only two groups under discussion -- upperclass women and and female commoners.

Only in or in only? - English Language & Usage Stack Exchange