This is important. In this timing, tori should wait until the attacker feels they have succeeded... and then enter, in whatever direction you decide. This is sen no sen timing done really well, in my opinion.

Of course, go no sen is evasion of some kind letting the attack spend it's energy and making your attack using tori's energy as it's finishing the attack cycle. When the timing is really fine, you can enter (irimi) tori's power structure while going backwards interrupting tori's back muscles firing to recover. This often is felt by tori as a small explosion or even the muscles seeming to "seize up" for an instant.

However, there is another timing, sen sen no sen, where tori is just deciding to attack and just as the signal is going on it's path to move into the attack, you move, which forces the attack just before it starts, or just as it starts but very little power has come into the attack. This is not really the common idea of a preemptive strike, but it's extremely powerful and upsetting to tori's mind. When done properly, tori is trapped into a recovery cycle before they can attack again.