Nagasu: 流 the meaning of kanji is flow or current (like a stream). Practically nagasu has ‘simply’ 2 meanings or ways to use. One is to guide the energy you are receiving. The other is to let the energy pass your body.
To guide
Nagasu is to guide the opponent’s energy, like the translation of the kanji, with a flow. You can guide energy into another direction. Remember that guiding (nagasu) and deflecting (inasu) are different things although they go hand in hand on many occasions. When you decide to guide the energy with for example the right arm the left should be used for attack, this is a proper use of nagashi when you consider guiding. This is very common in kihon kumite.
To pass
Nagasu is also known as to let the attack or energy pass your body. Easily said, when you are ‘not there’ the energy passes the body. However, that could be 2 things: to escape and to avoid. While both might have the same meaning or intention, for karate avoiding (kawasu) is more proper as this means that you are within maai to hit the opponent instantly. Remember that you are ideally hit in the center when practicing, but you might be hit on the right or left side of the body as well. Depending on this, move in the appropriate direction.
PRINCIPLES