The idea works on the observation that volume which has been sitting in the depth for some time ('old' volume) will tend to attract the price towards it, especially if it is large. Conversely, 'new' volume tends to 'push' the price away from it. If the new volume is added to the top 3 bid lines, the price appreciates, but only if the amount of volume added to the top 3 ask lines is not also increasing. And vice versa.