(A) Notes for the Attacker
1. From shizentai (natural position) step back with the right leg into hidari zenkutsu-dachi (left front stance) with hidari gedan-barai (left lower level block). Simultaneously rotate the hips back into a proper `to the limit’ hanmi (half facing position). Announce “jodan” (indicating a prearranged ‘upper level’ attack). Note: when assuming hanmi naturally bend/contract the rear knee so that it is ‘under your hips’ and that the hips remain level.
B) Notes for the Defender
1. The defender waits in shizentai for the attack to come, then reacts `after it is launched’ even though it is prearranged (note: pre-emptive defence invalidates the drill). After the attack is on its way the defender drops their weight vertically whilst stepping back with the right leg into hidari zenkutsu-dachi. Concentrate more on dropping as opposed to stepping back so as to maintain an effective ma’ai (distance). Simultaneously twist the hips into a full hanmi – to the limit with hidari jodan age-uke (left upper level rising block). To counterattack thrust the rear leg straight rotating the hips back into shomen with migi gyaku-zuki (right reverse punch). Remember that that hips slightly precede the launch of the punch (so that the legs and waist are the main source of power). Remember: Traditional karate primarily utilises the kahanshin (lower body) as opposed to the johanshin (upper body).
To conclude I hope that these notes assist you with your Kihon Ippon Kumite training! Obviously, I've just covered the very surface, but like everything, the initial points are what determine the effectiveness of the more advanced practice.