First, you have to understand how the NAND gate works. When both inputs are positive (high), the output is low. When either input is low, the output is high. In the diagram below, if both A in and B in are low, both outputs will be high. That is not a useful state. If both inputs are high, then A out and B out cannot both be high and both outputs cannot be low either. So, one has to be high and the other low. You don't know which is which unless you force it by taking either A or B low, then you know which output is high. OK?