That is torque converter.
eg. whatever gear you are in, let's say you are
- leisurely cruising, 80kmph @ 2.5k rpm.
- when you want to accelerate, it become 80kmph @ 3k rpm.
- when you free fall, it become 80kmph @ 1.5k rpm

And no, it is not dropping gear when accelerating (an example of down shift with the context above it is 80kmph @ 4k rpm)

There are some free-play or 'follow-up' work that the torque converter has.