To pull together several valuable posts on this topic, not to describe it’s use in Isshinryu, per sae, but to understand what other Okinawan systems use.
The use of the kicking foot toe is varied between the various Okinawan systems. From Uechi Ryu with it’s notable Chinese origins, to the more subtle uses in Goju, Matsubayshi and Isshin Ryu’s to name a few. This is an attempt to pull together several comprehensive resources on the topic.
Beginning with Toudi references in Itoman Seijin (Morinobu’s) book Toudi-jutsu no Kenkyu It is interesting in that there is no mention of toe kicks. It does describe kicks but dosen’t talk about the kicking surface for same.
Types of Kicks
Rooster (Crane Standing on One Leg)
The rooster kick takes its shape from a rooster or crane resting on one leg. When you kick with this technique you do not kick up with the leg straight but instead bend at the knee and make contact with the foot or shin. This method of kicking is quick, powerful and stable.
This method kicks upwards with a slight rising motion.
In this method of kicking the leg is deeply thrusts outwards.
Itoman does say his description is lacking hundreds of other techniques, so this wasn’t intended to be a complete listing by no means. The translation was done by Mario McKenna.
Christopher Caille of www.FightingArts.com has written a bit toe toe kicks in his on line magazine.
The Old Okinawan Karate Toe Kick: Part 1- Introduction & Execution
The Old Okinawan Karate Toe Kick: Part 3- More Shorin-Ryu & Intro To Naha-te
Engami Shigeru (Who went from studying Shotokan with Funakoshi Sensei to creating Shotoka) described earlier toe kicking practices from an article on my blog.
Engami Shigeru description of these toe kicks it’s found in his work “The Way of Karate Beyond Technique”.
An example of the Matsubayshi Ryu Front Kick
An example of the front toe thrust found in Hiagonna Morio’s Goju Seisan Kata.
This is not meant to be a complete listing of Okinawan kick potential, however it may be useful in understanding Okinawan technique potentials.