Equivalent formula for LTL with and without past operators
The "equally expressive" statement means that if a formula of PLTL is a statement about the future, i.e. if it's evaluated at the first instant $0$ of the time domain $\mathbb N$, then there exists an equivalent LTL formula.
This means that nesting future and past operators is not more expressive than nesting just future operators, as long as the global meaning of the whole formula is solely about the future.

Here are some examples of this equivalence: 


$\lozenge\square\lozenge^-\varphi\equiv\lozenge\varphi$ 
$\lozenge(\alpha\wedge (\varphi \mathcal S \psi)) \equiv \lozenge (\psi \wedge (\alpha\vee \bigcirc (\varphi \mathcal U (\varphi\wedge \alpha)))$