Intel microarchitecture code name Sandy Bridge introduces a new decoded ICache. This cache, called the DSB (Decoded Stream Buffer), stores uOps that have already been decoded, avoiding many of the penalties of the legacy decode pipeline, called the MITE (Micro-instruction Translation Engine). However, when control flows out of the region cached in the DSB, the front-end incurs a penalty as uOp issue switches from the DSB to the MITE. This metric measures this penalty.
A significant portion of cycles is spent switching from the DSB to the MITE. This may happen if a hot code region is too large to fit into the DSB.
Consider changing code layout (for example, via profile-guided optimization) to help your hot regions fit into the DSB.
See the "Optimization for Decoded ICache" section in the Intel 64 and IA-32 Architectures Optimization Reference Manual for more details.