Well, when feared, model of player ignores collisions with other models (like buildings), it only accepts collisions with terrain. You probably noticed that when feared, you dont fall under terrain. But blink just moves player model from location to location, which means no collision is generated which would be there if moved normally. Mangos still cant do a correct check, whether the location, where model is going to be, is a valid location (over terrain). That way you can get under terrain, while its not possible when feared.