Newbie question.  Sorry, but I did search.  I dont' have the proper terminology so that makes it more difficult.

I bought a P3S in May.  I flew it for a while and realized I wanted more range so I purchased a P3P.  On the standard, the motors shut off when you hold the left stick down.  It sort of clicks into a downward position.  On the Pro, that isn't the case.  You just hold it down for a few seconds and the motors shut off.  My question is will it only shut off when on the ground?  Do I have to worry about it shutting off if I am flying downwards for too long?  And I guess the last question, can I disable that so I have to use both sticks to shut off?

Motors will shut off only on ground when stick is max down. In the air you can shut off the motors only in case of emergency with sticks in corners (like starting)- do not try this-

ok great.  thanks

The left stick can be held all the way down and it will not shut off the motors while flying. They only shut off after a few seconds while the aircraft is on the ground. You should stop the motors that way. If yo do a CSC to shut the motors off sometimes you'll get a throttle jump and I've seen people's Phantoms tip over if it's windy or on uneven ground.
So just use the throttle to shut down after landing.