Coyote time is just a name given to the ability to still jump while off of a platform. Think og it from the coyote and the road runner old cartoon where he would walk off a cliff, then look down, then fall. Unity can be done a few different ways, the main way is like you said, extend platforms, or add a timer that checks if you are off a platform, usually 0.5f seconds. if you jump with in that time, you can still jump, if you don't jump you just fall.