"normal flight attitude" smells to me like the best equivalent for "ligne de vol" in the BEA report.
In addition, one translation of "accélération" is "G"; thus "avec forte accélération" becomes "with high G forces". Source for this is Dassault's house dictionary:
Lexique bilingue. Members may want to rummage through other equivalents for the term.
Nonetheless, Harry Mann is convincing with:
Airplane is travelling down. It makes contact with the water which stops the fall. This causes upward acceleration (possibly to a zero vertical speed).
Strong vertical acceleration means a strong CHANGE in the vertical component of the velocity. It doesn't necessarily mean anything beyond that.
Hope this helps.