Energy Flow Direction

Half arrow indicates the direction of power when $e\cdot f>0$.

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Example

Think of it if submodel 1 is a spring, submodel 2 is a wagon and the Effort and Flow is $F$ and $v$.
If the force is in positive $x$ direction, then $ef>0$, therefore the energy flow direction is from submodel 1 to submodel 2.

Negative power if

$$\begin{array}{rl}F>0,& v<0\\ & or\\ F<0,& v>0\end{array}$$

etc.

It is quite logical, just think of the formula $P(t)=e(t)f(t)$.