Trading at the bid-ask midpoint and hidden orders

[backtothebeach]

I've had mixed success trading between the bid and ask. On liquid stocks, I often find a hidden order at the midpoint executes, I suppose the counterparty has the opposite hidden order at the midpoint? 

 

IB's midpoint algorithm on the other hand can work, but it feels like the algos react to a larger order and move the price so the midpoint gets worse and worse.

 

Frankly, it is all a bit of a mystery. For example a round lot (less than 100, e.g. 50 shares) order on FFH.TO does not change the bid-ask at IB on my screen, but I would guess the full order book is visible to other market participants. 100 lot hidden orders on FFH can execute, round lot hidden orders never execute.

[backtothebeach]

What if someone mentioned on a message board that they had an active hidden sell order at the bid-ask midpoint on a thinly traded stock like ELF.TO? Would that be legal? Asking for a friend.