Embusen is literally ' the performance line'. The word 'enbu' means performance. Sen is line. This is simply the line that the person follows when he is 'performing' the kata. For example, in Pinan Shodan the embusen first goes to the left, then to the right. then forward. From that point it goes diagonally, and so on. This line that the performer traverses is the embusen.
 
Seichusen is a concept that is fundamental to Wado. Seichusen - sei:correct/chu:center/sen:line. Correct center line. This is the imaginary line that the attacker's punch or kick will travel as it goes towards the opponent. Conversely, this is the line that the defender must defend against because the attack is coming at her through this line. As long as you guard your seichusen you will not be vulnerable to the attack.
 
In a living kata, the embusen and seichusen are one. As you move along the embusen you defend and attack through the seichusen. When you perform your living kata, your seichusen must be as narrow as feasible. When you perform your kata, your movements must not be telegraphed, your body must move as one without leaving parts of it behind (when you go, you go) and when your body settles at the end of a movement, it never gets in a position where your center of gravity forces you to be stuck to the floor (itsuki). All of this together makes for a living kata, ikita kata. 
 
Hironori Othsuka