Question about 2nd gear "lag"...

When I take off, say, at 1/4 or 1/2 throttle, and the car goes from 1st to 2nd... once it hits second it feels normal, then for a second or two it seems to drop down lower in the rpm band, almost like the torque converter is locking up. What is this? Why is it setup this way? Is anyone else experiencing this? I thought I read about it somewhere but couldn't find it when searching.