I believe that BEGIN_TEXT and END_TEXT are just easier-to-read replacements for TEXT(EV3(<<'EOT')); and EOT. You should also be able to put Context()->texStrings and Context()->normalStrings before and after these commands.
To explain a little what is happening here, suppose $f=Formula("x/3"). Then $f is a complex hash of information, including how to represent $f as a string for say other math objects, as well as how to represent it as a string to be processed with tex.
By default, Context()->normalStrings is in place, and if you had $g=Formula("$f+1");, then $f is replaced with x/3, and you have something equivalent to $g=Formula("x/3+1");. But for display to students, you want $f to come out as \frac{x}{3}. Setting Context()->texStrings makes this happen.
Lastly you go back to Context()->normalStrings, because if you now used $g=Formula("$f+1");, you'd get an error from $g=Formula("\frac{x}{3}+1").
I've used this example with division to reveal a big difference between the two modes. But in general, "normal" strings and "tex" strings are just different.
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