A mass rush is when a very large snake gets killed and the smaller snakes are speeding and wind up getting killed by all the commotion. Mass rushes are very dangerous and usually lead to many more snakes getting killed than the one which was originally killed. The best strategy for mass rushes is to use the boost only when you have a lot of room. Also, if there are too many snakes in a mass rush, don't join in, until some of the other snakes die. Also, if you are a big snake, you can trap all of the snakes in the mass rush, since they will be focused on the remains and may not be able to escape in time.
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