Video Example: Sensei Kunio Miyake, 8th Dan of Shuko-Kai-International, explains the difference between Go-no-sen, Sen-no-sen and Tai-no-sen (Sen-sen-no-sen) tactics in the kumite.
Go no sen
Go no sen is block and attacking the opponent after their attack has finished. For example, my opponent throws a reverse punch at my face, I stop that attack with a block and then follow up with a counter attack.
Sen no sen
Sen no sen is a block and counter attack while the opponent is attacking. For example, my opponent shifts in with a reverse punch and at the same time, I shift back, block his reverse punch with an outward block and counter attack with a reverse punch at the same time. The timing here is very important because both people are committing to the attack at the same time.
Sen sen no sen
Sen sen no sen is attacking and counter attacking the opponent while they are attacking. How I understand this is, make opponent shifts in and attacks with a reverse punch and you shift in and attack with a jab at the same time and immediately follow up with a reverse punch.
The concept behind deai is you sense your attackers attack coming and you counter attack before the opponent can fully start his offensive attack. For example, both opponents are either shifting or standing in kamae and your opponent goes to throw a rear leg roundhouse kick, before he can get the roundhouse kick off, you shift in and jab.
The key here is that your opponent gives some sort of tell or sign (leaning before kick or changing balance points before bringing their leg up) that they are going to attack and you sense this attack coming and counter attack them before they get their attack off. This tell or sign could be a twitch, eyes widening or even being able to feel their energy before they attack.