A week of recovery on the local trading scene was turned topsy-turvy last week when the market learnt that regulators had raided four brokerages and begun questioning several remisiers.

The news was a stark reminder to local investors that as calm as the market might look, there are many dealings going on underneath - some shadier than others.

While details on the investigation are yet to emerge, sources told The Straits Times that the remisiers are being questioned over alleged manipulation of stocks.

The word going round is that the Commercial Affairs Department is cracking down on a "pump and dump" syndicate, where a group of remisiers rotate the shares around themselves to inflate the price.

That happened during the October 2013 penny stock crash, which also triggered a series of regulatory raids. With that scandal still fresh in investors' minds, it seems rather baffling - appalling even - that a group of professionals would try to rig the market yet again.

However, history has shown such attempts are far from uncommon in stock markets worldwide.

And the fact that this fresh scandal is unfolding so soon after the penny stock collapse is a testament to how vital it is for regulators to stay ever vigilant.

The incident serves as yet another reminder to retail investors that they often seem to be on the losing end.

Professional traders often act as playmakers in the market, implicitly directing the hottest trading trends of the day through their own purchases and sales.

Some retail investors watching a small stock rise and rise are unable to resist the lure of a quick profit. But go in at the wrong time and they could lose it all in one fell swoop.

If there is a silver lining to all this, it is that the regulators have shown they are willing to take real action against market manipulators.