Immersion involves loss of self-consciousness; a yielding to the moment.
Yes, we need some sense of self in order to function, in order to survive.
But it can also be a major impediment.
Instead of being one with the event, people tend to get caught-up in speculation, doubt and the avoidance of negative possibilities.

To become immersed, we must detach ourselves from thinking and pay attention to what is happening.
To what is right in front of us.

In class this process occurs whenever we undertake any kind of partner work or self defence training.
There is no scope for thinking.
We must become body-centred and present.