Which fund? The average holding period of the Medallion Fund is allegedly 2 days. That is not conventional momentum investing. On what do you base your assumption that Renaissance pursues momentum investing? They have never disclosed strategies for the Medallion fund. Do you mean some other Renaissance fund?
There are always the same number of buyers and sellers! It's a market/auction. It can be that the market maker is acting as seller in an unbalanced market but they are still a seller, it's 1:1. What drives the market up is buyers buying at higher prices and sellers selling at higher prices. Supply/demand
these two methods are the most common to define a signal of an impulse strategy -a fast exponential average coming out of a slow period band - a fast period band leaving completely from a slow period band (method similar to the previous one but with an extra safety margin
Of course, the problem with trading "momentum" is that your technical study or statistical sampling model has to, by definition, sample a considerable data population and it is a lagging indicator. Point being, by the time you get in it might very well be too late. This is the ONE example where a trader with savvy and experience can do well with an order book DOM.