Official Webpage of A.Tanzadeh, 8th Dan, Kyoshi 糸東会

Go no Sen, 後の先 (post-initiative)

After the attack, block/evade and counterattack.

The ‘Go’ (後) in ‘Go no sen’ means ‘after’. Quite literally, immediately after you’ve been attacked, let’s say with a punch, or indeed a flurry of punches – which you have effectively blocked/evaded – you counterattack.
Go no Sen is not just counter-attack. Go no sen is a mental state, a level of concentration assumed during combat. It is more correct to say that it is harmonization with the very movement of attack, not just defense.