Hi
A prospective reserve formula for a conventional with-profits policy could look like:
Value (Sum assured + Bonsuses declared to date) + Value (future expenses) + Value (future bonuses) - Value (future premiums).
So, the bonuses already declared are treated like the sum assured (as these are both guaranteed benefits). The future bonuses assumptions will need to be in line with TCF. To set these assumptions, the company might project asset shares.
In a simple case, the prospective reserve is then rather like projecting asset shares and then discounting them back (ie asset shares are projected, they're turned into bonus rates that determine future benefits as being set by asset share, and these benefits are then discounted back). That's why the retrospective and prospective approaches are equivalent.
(In practice, likely to actually use a cashflow approach rather than a formula, but doesn't change the logic).
Not sure if this is quite answering your question?

Hope so, but do say if anything's not clear
Lynn