I read the mechanics book but it is a little confusing. Please help me understand. Under what conditions do we lock down.

Bart,

You keep rotating at the end of each period until a new possession is gained and the shot clock will be turned off (under 30 seconds). AT this time the "lock down" will go into effect.

That is correct...but let's also remember the new mechanic this year of locking down anytime there is 5 seconds or less on the shot clock...that's gonna be fun to remember!