It means that you can influence the result by controlling the first part of the input. In other words, you can find an x such that H(x) == H(y) (for x != y) if

It means that you can influence the result by controlling the first part of the input. In other words, you can find an x such that H(x) == H(y) (for x != y) if you get to control the beginning bytes of x and know (or control) the beginning bytes of y.