What does the expression "here falls the shadow" mean?
The context is a list describing how to teach your kids to become entrepreneurs:
6. Teach the mental nexus. Here falls the shadow. Rational people do not become entrepreneurs [...]
Strictly in your specific context, "Here falls the shadow" is something like "Here is the problem".
Culturally, shadows easily refers to darkness, to something black and depending of the context, something bad, something that goes wrong.
What your sentence is bringing up is that irrational people are those that become entrepeneurs. They should be madness, creative; they should think outside of the box to make big deals and here is the problem: people commonly are too rationals to go off the line.
I believe "Falls the Shadow" is a reference to the uncertainty of an entrepreneurial life and possibly a reference to T.S. Eliot's The Hollow Men