This is our basic learning method for shomen-uchi ikkyo; aikido's first principle and one of the first techniques required for grading.
The fundamental lesson of ikkyo teaches us how to enter directly into an attack. At its most basic level, the tori (defender) practices getting off the initial line of attack and then returning to it in order to take an attacker's balance.
While there are many reference sources for this waza–as well as interpretations as to how it can be adapted to other attacks–the most evident historical application of ikkyo involves tori seizing control of an attacker weilding a katana (Japanese long sword) and attempting to cut a shomen-uchi (strike to the forehead). This is the foundation of the waza, however from this fundamental application we can explore various armed and unarmed attacks, as well as modern-day adaptations.
The most important thing to consider is that ikkyo is not solely a technique for dealing with a frontal attack, but rather it is a method of "piercing" through any attack, with the goal of making a direct connection to uke, and thereby rendering them unable to successfully continue their attack.
Demonstrated here, we break down the steps and timing required to circumvent the initial attack. Beyond the kihonwaza, students learn how to later manipulate the attack line and–eventually–take uke's center directly from the attack line.
As always, comments and questions are welcomed below.