Quote from rwk:
I was reading through some old related threads on ET, and that triggered a realization. I think what I overlooked is that the hidden size is probably coming from stops being elected. There could be new orders arriving, but more likely it is from stops. When a stop is elected, in most cases it becomes a marketable limit order immediately (i.e. with no change in the timestamp).
When a stop is triggered, it would add to the pressure but here someone provided additional liquidity (absorbed a stop order), so the additional liquidity is not caused by stops being triggered but rather by them being aware of the situation (and possibly anticipating a reversal).
I don't necessarily agree with ofthomas that you can only use this info if you are LL/ULL. This info can be used even if you can't act on it in that precise moment.