Legs and feet are very important for stability and locomotion. With them we can run, walk, jump, and kick. It is also important for self-defense in order to: run away, increase or decrease distance, and hit powerfully opponents. Bujinkan Budo Taijutsu has several kicking techniques (what also depends on the school), and most of them are well-known.
Reading Ten Chi Jin Ryaku no Maki (the book of heaven, earth and human-being strategy) of Hatsumi Sensei, many basic kicks and counterattacks can be found. According Hatsumi Sensei, the knee has a very important point on kicking techniques and you can practice pulling it up to chest and holding the knee with both hands before practice basic kicks.
There also are two important points to practice kicks. First at all, avoid to stretch completely your leg and stuck the knee violently, it could injury you. Second: along the movement drop a little bit your hip. It is also suggested to train kicks pointing feet fingers to your own body (as beginner practice) along the performance.
Several coordination exercises should be practiced. Robert Renner Shihan of Bujinkan Zero Point Dojo produced DVDs that describes very interesting exercises to improve the control and coordination of whole body. It improves the Taijutsu.
Recently, I watched “Saigo no Jissen” movie again and there Hatsumi Sensei shows some pictures of Takamatsu Sensei describing how to hit with hands and feet.
Like everything in martial arts, kicking is connected to posture, distance, and timing. Usually, timing and distance are considered. However, if you are not well positioned (bad posture or bad distance, for example) your kick will not be effective. A simple test to do is to kick a punching bag assuming a balanced, but not structured posture.
During solo training, prepare and coordinate all of aspects of the kicking techniques. A punching bag could be used. Practice dozens of basic Bujinkan kicks, feeling different angles (controlling distance), weight changes, and coordinating all movement phases.
In order to kick effectively, always regard 急所 (kyusho, vital/vulnerable points) during practice and research. There are several points on the body, but each one could be attacked on a specific way, and with specific hit. In this case, a specific kick. When a vulnerable point is attacked, opponent assumes a specific posture as body reaction. That is also the reason you should practice and understand it in order to control the flow of the combat. Always moving one step ahead without forget the balance.
When practice with partner, it is also important to know how to react after to be kicked. Several Ukemi waza (receiving techniques) are well-known for these purposes and should be practiced deeply. Still, do not forget receiving techniques with weapons.
Some weapons could embarrass default kicking movement (while kicking or after that). Sometimes, it requires adaptation. Even when you are using a weapon and receiving a kick, the receiving technique probably will need dynamic adaptation. Go ahead. Try.
Try to explore all kicking possibilities, regarding all variables related and improving your Taijutsu. Kicks are very important to improve self-defense understanding and body perception.
(Ninja no Kokoro Ninpo Ikkan)