There is no mechanism by which an average of past prices exerts force on the market. Any effect comes entirely from attention: enough participants place orders near the 200 day average that real orders exist there, a mild self-fulfilling-prophecy.
Because of that, the effect is strongest on the most-watched averages on the most-watched instruments, and essentially absent on an obscure 37 period average of a thinly traded name.
The honest way to use it is as a zone of interest rather than a line, and always with a stop. In a genuine downtrend the 200 day average is broken without ceremony, and the number of traders who bought it on the way down is precisely why the decline had buyers to sell into.
Related: self-fulfilling-prophecy, simple-moving-average, support, round-number-level, polarity-principle