Most of the no-think RL gain in hybrid reasoning models is thinking leakage: erasing an 8-D think subspace at 8B removes 75% of it
Training Qwen3-1.7B, 4B and 8B with GRPO in no-think mode raises accuracy while responses grow longer, carry more reflection words, and internal activations shift toward the original model's think mode. Weight scaling, activation patching and attention-head interventions indicate that much of this gain comes from reactivating reasoning the original model already had rather than learning new reasoning behavior; at 8B, replacing an 8-dimensional think subspace removes 75% of the gain, and the reverse exchange transfers 50% of it to the original model.