I will try.
If zone A is the "trigger" zone:
BI can monitor an object for example in zone G and when that moves inside zone A then it triggers if all other conditions is meet, faster detection because the object is already monitored. Similar as zone crossing but it only checks so the objects get into zone A so it can also start in zone A. With only zone crossing (A-B) or similar it can be hard to monitor a lawn and get good crossing lines this way you only define a zone and have the rest as a diffrent zone instead of masking it out.
The best way of using this would be if you have a bush or something in the garden that you now have masked out because of false alerts. Instead of masking it out you can put zone G around it and the rest of the picture as zone A (expect the bush), this way an object (the bush) moving inside zone G will not make any triggers but if someone walks across the bush or similar BI will still be monitoring the movement in zone G instead of losing the object, or if someone comes out of the bush it will already be monitored and when moving to the A-zone it will trigger, hopfully get a faster trigger or make it possible to have longer triggering times which often means less false triggers.
Maybe a poor explanation but I hope you get the idea.
