There's an example in the pdf that would clarify. Note that the pdf has no pictures, but
you can pretty much tell what's going on:
The Storm L ance declares a charge targeting E xemplar A.
The Storm L ance then moves in a straight line toward his
target. After moving 4˝, he stops short when he moves
into base-to-base contact with E xemplar B. H e then makes
impact attacks targeting E xemplars B and C since both
models are in his Mount’s melee range.
After hitting and destroying E xemplars B and C, the Storm
Lance continues his charge movement to E xemplar A.
If E xemplar C had not been destroyed, the Storm L ance
could have continued the charge anyway, although he
would have suffered a free strike. If Exemplar B had not
been destroyed, the Storm L ance’s charge would have been
unable to continue, and the charge would have failed.
Note that if you could move through the model, it would say so (like trample).
Instead, you are still constrained by regular rules unless the rules give you an exception.
Compare the Impact attack wording with the following wording on trample attacks:
[The trampling warjack] moves through
any small-based model in its path, but there must be room
for the trampling model’s base at the end of the movement