In keiko your objective is to break your opponent’s kamae and strike a target as soon as you have created an opportunity. There are many ways to do this you can make him start an attack and strike as he begins his movement. You can use kaeshi waza, suriage waza or nuki waza to counter the attacks that you encourage him to make. You can take his centre by stepping in and making a strong tobikomi attack, or you can use your shinai to knock your opponent’s weapon up, down or to the side, even to twist it out of his hands. All of these are permissible with the rules and spirit of kendo and you should do them as energetically as your stamina will allow
It is equally permissible to move your opponent by striking with your body, but only in the form of correct tai-atari where the contact should be tsuka on tsuka with the hands at waist height and the power coming from the lower body. Pushing to the chin or face, using your feet to sweep or trip, trapping your opponent’s shinai or using your own to push any part of his body constitutes an infringement. Taiatari should be one quick body check followed by an attack rather than a long concerted pushing match.
Tsuki is a valuable kendo technique but must be done correctly as a sharp on-off attack. Mukaetsuki, with your arms locked as your opponent makes a forward movement against you is considered the height of bad manners. Even a good attacking tsuki against a teacher or senior in poor taste, if it is done when they make an opening for you to hit men.
Most of these are obvious violations of the rulesa of kendo and would be penalised in shiai. There are other less obvious breaches of etiquette that are undesirable in keiko. Using your shinai to block without countering is wrong and spoils the flow of the tachiai, as does starting and stopping an attack mid flow to prevent your opponent from hitting you. Hitting your partner off-target in order to create an opening is equally bad, as is showing contempt by celebrating or walking away after striking.
Some of the rules invariably get bent in shiai, but there are 3 referees in the court to stop you from transgressing too much. In keiko it is up to you to train as hard as you can whilst still showing respect for your dojo mates.