crosscheck

The cabin crew are responsible for the cabin and once the cabin is secure they will let the flight deck know...in the old days they turned a "flipper" on the centre pedestal on the flight deck, now they call and usually the PNF will do this. On all (most ) check lists for commercial ac Before TO Checks have listed Cabin...Secure. This verifies all overheads bins are closed and the pax are sitting all strapped in.
Additionally before taxi an announcement will be made for the cabin crew to arm the door (meaning that escape slides are primed for evacuation) and crosscheck. Crew arming 1 door left will check 1 door right and vice versa. Everything in aviation as I am sure you are aware juding by your name is crosschecked! A pilot never completes the checklists without confirmation of settings from the other operating crew this is similar for cabin crew....I should know I live with one!
In short before push and start doors are armed and crosschecked as once the ac is moving this needs to be done ready for evacuation. Cabin secure (CC take seats for departure) is done prior to entering the runway...in most cases! All bins are closed and pax/cc seated.

Thanks for that valuable peice of information PH - Good stuff to understand.