Basically it will underclock/shut down unneeded cores and bump up ones that are in use. However I have seen some anecdotal info that states that there are two stages. Stage 1 is all cores up to a certain frequency (depending on the TDP of the cpu) and the other is half the cores up to max turbo.
What could be causing the problem is this. Lets say that the game takes 4 cores. You have 8 cores and 4 FPUs (which are used in gaming). With turbo on the BIOS senses that only 4 cores are in use. So it shoves everything on to 2 modules, and powers down the other which frees up TDP for pushing the 4 remaining cores faster. since it shuts down a full module, you lose 2 FPU and are down to 2.
When you disable turbo core what happens is that the game sends out a thread for the FPU. You have 4 available so those threads have less time to wait to be processed. So the scheduler utilizes all the resources it can to get the best performance.
I'd say OC the CPU over enabling turbo core. OCing would help utilizing all cores at more speed whereas in turbo, as described above, your CPU will run only certain cores on OC and others will be down.