pic by Steve
Firstly kakarigeiko is "attacking practice". It is designed to build stamina and the ability to break through against a powerful opponent. It should only be practiced once you are completely warmed-up, as the intensity of it can cause injuries if the body is not ready.
Generally kakarigeiko is performed in such a way that the motodachi holds centre and the kakarite 掛手 (person who is attacking) must create an opening before attacking. Kiai should always be very loud and extended, continuing with the zanshin: no short yelps or grunts. This helps open up the lungs and increases cardio-vascular (heart and lung) fitness.
It should be done will nothing less than 100% of your energy and willpower. This 100% will increase as you go through repetitions, because kakarigeiko creates its own kind of warm-up. Imagine a battery that, the more you use it, the more power it can store: that's the human body. Subsequent repetitions will be performed at higher intensity, so that your initial energy output will seem small in comparison. Of course after enough repetitions this energy curve will start to dip rapidly. It is common for kakarigeiko to be used in Japanese uni and high school training as an especially severe training method, where kendoka are driven to complete exhaustion and even collapse.
In the early stages kakarigeiko is primarily a fitness-building exercise. That means it should be performed at your maximum energy level. As you get fitter over time, that output level will increase. Emphasis is on using your lower body to drive through and beyond your opponent, turn quickly and commence a new attack without stopping or pausing. Motodachi should encourage mostly atttacks moving forward: men, kote, do, ni-san dan waza. At this stage motodachi should not deliberately hamper their training partner.
Later, when fitness and skill levels are higher, motodachi can perform some actual blocking of cuts. Motodachi can also block their opponent with their body forcing taiatari and hikiwaza. This is a higher level of kakarigeiko where the kakari-te has to overcome the frustration of being hampered, or of their attacks miscarrying. Mukaezuki,(迎え means 'to welcome') where the motodachi holds centre so that even when kakarite thinks there was an opening, s/he runs into the tip of motodachi's shinai, can also be used by motodachi but it is potentially dangerous and should only be used by experienced dan grades.
More difficult still is ai-gakarigeiko 合掛稽古, where there is no motodachi and both kendoka attack simultaneously. Without a high level of regular training and skill, this drill can be ineffective.
The main mental benefits of kakarigeiko are: 1) to instill a spirit of perseverance ("knocked down seven times, get up eight"), and 2) to enable us to realise a state of mushin during training. Obviously in order to make a successful and appropriate attack there is some degree of planning and analysis required. However during kakarigeiko we learn to trust that this can still happen outside our conscious thinking processes as the sequence is too rapid for these processes to occur. So usually it is during kakarigeiko that the kendoka first has an experience of mushin 無心. They find themselves doing something correctly first, then only think about it later.
Yoshiyama Mitsuru sensei, who has written an excellent guide to passing high-level kendo gradings, says he feels kakarigeiko is of more benefit to one's kendo development than even jigeiko.
Kakarigeiko is the last drill before jigeiko. It has almost all the dynamism of jigeiko, and the least structure of all the drills. The main thing people find difficult, apart from the huge amounts of energy required, is the improvisational nature of it. When you get it right, it's like all your cuts are predestined. When you don't, it's like you can't put a foot right. But it takes some trust in yourself and your training to allow yourself to find rhythm. The best way to find that rhythm is to go all out and hold nothing back. Put most of your power into your legs, and keep your upper body relaxed and your cuts light. And sound like a homicidal maniac. Really.