The true merit of it above all previous methods of concentration and containing was that, whether, as planned or as delivered, it prevented the enemy from knowing on which part of their line Nelson intended to throw his squadron, just as we are prevented from knowing to this day.

'They won't know what I am about' were his words to Keats.
The point is clearer still when we compare the different ways in which Nelson and Collingwood brought their respective columns into action.

Collingwood in his Journal says that shortly before 11 o'clock, that is, an hour before getting into action, he signalled 'for the lee division to form the larboard line of bearing.' The effect and intention of this would be that each ship in his division would head on the shortest course to break the enemy's line in all parts.

It was the necessary signal for enabling him to carry out regularly Howe's manoeuvre upon the enemy's rear, and his object was declared for all to see.[37] Nelson, on the other hand, made no such signal, but held on in line ahead, giving no indication of whether he intended to perform the manoeuvre on the van or the centre, or whether he meant to cut the line in line ahead.

Until they knew which it was to be, it was impossible for the enemy to take any step to concentrate with either division, and thus Nelson held them both immobile while Collingwood flung himself on his declared objective.
Nothing could be finer as a piece of subtle tactics.