Breakneck describes speed that is very fast and dangerous — fast enough, the word suggests, that you could break your neck. It goes in front of a noun and almost always sits with a small group of speed words: breakneck speed, a breakneck pace, breakneck velocity. 'They drove down the mountain road at breakneck speed.' You will also see it about work and change, not just movement: 'The company grew at a breakneck pace', 'reforms coming at breakneck speed'. There the danger is not to your body but to your health, your quality, or your ability to keep up. That extended use is now probably the more common one in newspapers. The word carries a real edge of warning. It is not simply a compliment about being quick — it hints that this is too fast, that something will go wrong. But it is also used with a kind of excitement, especially about films, chases and races, where the danger is part of the fun. Note that it is an adjective; you cannot use it after 'be'. 'He drove at breakneck speed' is right; 'His driving was breakneck' sounds strange to most speakers, though writers do occasionally stretch it that way.