It's done by re-mapping the standard ASCii character set to that of Unicode by using javascript functions. It's not really upside down it just creates the illusion that it is.
Depending on which browser you are using and what charset is in use will decide exactly how it appears. On some less compliant browsers it simply doesn't work at all. Some very interesting effects can be obtained by using two or more iterations, for example, a one pass inversion with lower case reduction appears thus:
ʇ! pɐɹʍɹoɟ sႡ!ʇ pәɐɹ uɐɔ noʎ ɟ! iʇʇuɐɹoɯd! sɐʍ ƃu!ႨәdႨs ʇoႡnႡƃʇ sɐʎႨʍɐ ! puɐ Ⴁәɐʎ ¿ႡnႡ ƃ!ɯuɐzɐ ˙әႡoႨʍ ɐ sɐ poɹʍ әႡʇ ʇnq 'ɟәႨʇs! ʎq ɹәʇәʇႨ ʎәʌɹә pәɐɹ ʇou soәp p!uɯ uɯɐnႡ әႡʇ әɐәsnɔq s! sႡ!ʇ ˙ɯႨɹәoqd ɐ ʇ!nʇoႡʍ ʇ! pәɐɹ ႨႨʇ!s uɐɔ noʎ puɐ sәsɯ Ⴈʇoɐʇ ɐ әq uɐɔ ʇәsɹ әႡʇ ˙әɐႨɔd ʇ!Ⴁƃɹ әႡʇ u! әq ɹәәʇʇႨ ʇɐsႨ puɐ ʇ!sɹɟ әႡʇ ʇႡɐʇ s! ƃuႡ!ʇ ʇuʇɯɐoɹd! ʎuႨo әႡʇ 'әɹɐ poɹʍ ɐ u! sәɹәʇʇႨ әႡʇ ɹpɹәo ʇႡɐʍ u! ɹʇәɐʇɯ ʇ,ouәsp ʇ! 'ʎs!ʇʌɹәu!n әpƃ!ɹqɐɯɔ ʇɐ ႡɔɹɐәәႡɔsɹ ɐ oʇ ƃ!uɹpɔɔoɐ 'p!uɯ uɐnɯႡ әႡʇ ɟo ɹoәʍd ႨɐәuɯuoɐႡd әႡʇ ˙ƃә!uɐpɹ sɐʍ ! ʇႡɐʍ pɹuʇɐupsәn ʎʇႨɔɐႨnɐ ponႨɔ ! ʇႡɐʇ әә!әʌႨq ʇႨonupɔ !
Re-mapping a second time appears as this:
¡ cdnuoႨt bႨvә¡әә taႡt ¡ cႨuod auႨacႨty uәsdnatnrd waႡt ¡ was rdan¡әg. tႡә pႡaonmnәaႨ pwәor of tႡә Ⴁmuan mn¡d, aoccdrn¡g to a rscႡәәarcႡ at cmabr¡gdә u¡nәrvt¡sy, ¡t dsәno't mtaәtr ¡n waႡt oәrdr tႡә Ⴈttәrәs ¡n a wrod arә, tႡә oႨny ¡proamtnt t¡Ⴁng ¡s taႡt tႡә frs¡t and Ⴈsat Ⴈttәәr bә ¡n tႡә rgႡ¡t pcႨaә. tႡә rsәt can bә a taotႨ msәs and you can s¡tႨႨ raәd ¡t wႡotu¡t a pboәrႨm. t¡Ⴁs ¡s bcusәaә tႡә Ⴁuamn mn¡d dәos not raәd әrvәy Ⴈtәtәr by ¡stႨәf, but tႡә wrod as a wႨoႡә. azanm¡g ႡuႡ? yaәႡ and ¡ awႨyas tgႡuႡot sႨpәႨ¡ng was ¡pmoranttı ¡f you can raәd t¡Ⴁs forwrad ¡t
Some of the characters remain unchanged from Unicode the second time because the ASCii conversion is one way on certain characters. In Unix (Linux) they are known as Dead keys since they cannot be re-mapped by conventional means so we end up with an interesting mixture of Unicode and ASCii characters.
It takes the original posters idea to the extreme but I find I can still read the latter text even though it looks a little like a mixture of Russian and Greek
