'The whole impression,' as Nelson put it, in case he was forced to attack from to-leeward, was to overpower the enemy's line from a little ahead of the centre to the rearmost ship.

He does not say, however, that this was to be 'the whole impression' of the intended attack from to-windward.

'The whole impression' there appears to be for Collingwood to overpower the rear while Nelson with the other two divisions made play with the enemy's van and centre; but the particular manner in which he would carry out this part of the design is left undetermined.
The important point, then, in considering the relation between the actual battle and the memorandum, is to remember that it provided for two different methods of attacking the rear according to whether the enemy were encountered to windward or to leeward.

The somewhat illogical arrangement of the memorandum tends to conceal this highly important distinction.

For Nelson interpolates between his explanation of the windward attack and his opening enunciation of principle his explanation of the leeward attack, to which the enunciation did not apply.