Fast Line Inspections® MAP1-M/L:

The basic principle is:

Measuring1 - pressurize - measuring2.
And this is all happening underneath one measuring belt.

So, you have two measurements, when the pouch is just under the belt and one just before the pouch leaves the belt.

And in between (entering and leaving) some pressure-time (Read: time to leak gas) and you can do a comparison of the two values to determine whether the pouch was leaking.

Therefore the measuring belt is a little longer to have more pressure-time between the two measurements.

As you can understand the MAP1 is less accurate than the MAP3 because the time pressure is supplied is shorter than with the MAP3.