ZID controller described a conflict event when he/she descended an air carrier into airspace that had just been released to another controller.

Narrative

I was working DAC IM sector and FLM HI sector combined at DAC IM. There was a lot of turbulence in the area at all altitudes. I had a Regional Jet at FL350 and had just completed a sector split with FLM HI. A couple of minutes after the split; the RJ requested lower; I assigned the aircraft FL270 and they started down. I then was asked if I wanted a break. During the relief briefing I referenced RJ descending for a better ride. After putting my headset away I saw the RJ start flashing conflict alert with another aircraft. I walked over as the new DAC IM controller was issuing a turn to RJ to avoid an aircraft descending to FL240 inbound to CVG. The controller at FLM HI stopped the descending aircraft at FL280 and separation was never compromised. After a short discussion I realized that I had descended the RJ into FLM HI's airspace after I had released the airspace to them. I should be more vigilant in the time just after combining and De-combining positions. Possibly some software could be added to make the altitude in a data block change color when a controller enters an altitude other than one in their airspace.