Your question is about help for zoom - but tensionhigh and tensionlow is nothing directly to do with the zoom feature in mugen 1.1.
Tensionhigh, is the same as vertical follow in mugen 1.0 and older.
But lets explain it in an easier way:
For example, lets use as a test, the value "tensionhigh=120"
When your character jumps, or is hit up into the air, or flies etc, it means that as long as the character stays 120 pixels away from top edge of your screen, the camera will not begin to move up and follow the character movement.
when the distance between your character and the top or "edge" your screen is less than 120 pixels, the camera will begin to move upwards and follow your character.
The higher the number, the more sensitive the camera will be and move up when you jump or fly or are hit into air etc.