There are several systems, but they all do lock the action.
The K-31s have a rotating portion of the bolt that turns on the inner portion of the bolt. As you retract the bolt handle, a lug rides in a cam slot to turn and unlock the bolt head.
If no one else posts first, I'll snap some pics.
Other systems, like the Blaser rifles from Germany, use a very different system of locking tabs (almost like ball bearings) which are forced out into recesses in the receiver when you close the bolt. When you retract the bolt, they drop back inside the bolt.