I'm not familiar with the term 'tween'. What does it mean?

Use of asEP_ALLOW_UNSAFE_REFERENCES will allow you to pass primitives to functions with &inout (i.e. a reference to the actual variable so it can be both read and modified by the function). Despite being unsafe, it is no more unsafe than passing primitives by reference or pointer to a function in C++. If you don't care about sandboxing the script environment and trust your script writers to have at least a little bit of good sense when writing the scripts, then you should be perfectly fine with asEP_ALLOW_UNSAFE_REFERENCES.

Without asEP_ALLOW_UNSAFE_REFERENCES you will have to pass the primitive values in two args, one for input and another for output, e.g.:

void foo(int inValue, int &out outValue)
{
  outValue = inValue * 2;
}

The &out in AngelScript translates to either a & or * in C++, but know that the value referred to in the argument will be uninitialized as it is meant for output only.

Thanks for the links to the tween classes. I understand the term now :)