Gravity is pulling on the bottom of the slinky at all times. There’s no need for it to “realize” it has been dropped. It’s not like we switch on or off the gravity of the earth. There’s a constant force pulling it down (gravity) and one holding it up (hand). If it wasn’t for the tension in the slinky the bottom would have accelerated immediately. Just like it happens when a solid is dropped. However, the force in the actual slinky is fighting gravity until the