Why Did It Gap? Mechanical Gaps vs Information Gapseric

This is part four of the ASX order book series. The last post covered how to read the opening auction. This one is about the question that, across the journals I read, separated the traders who consistently made money off the open from those who didn't:

Why did it gap?

Most gap trading advice treats gaps as a pattern. Gap up, look for "gap and go." Gap down, look for "gap fill." The traders who did this for a living cared much less about the shape and much more about the cause.

Two kinds of gap

An information gap is the market repricing. A mechanical gap is the market's plumbing. If you fade an information gap you're arguing with new facts. If you chase a mechanical gap you're buying from, or selling to, someone who had to transact and didn't care about price.

Mechanical gaps on the ASX

These are the mechanical causes that came up across the journals.

One large order in the auction. A stock is indicated 7% lower and, when you look at the book, it's a single big sell order sitting in pre-open. No announcement, no sector move. One participant needs out and has decided to dump into the auction. Once that order is filled, the reason for the lower price is gone. Several traders described this exact setup as a favourite long: gap caused by one seller, no news, no fresh offers above, solid bids below.

Index rebalances and deletions. Index funds have to transact at the close on rebalance days, regardless of price. Huge volume can hit the closing auction and push the close away from fair value, and the next open can partially undo it. (There's a special topic post coming on rebalance days.)

Ex-dividend. On the ex-date the price is expected to fall by roughly the dividend. In a liquid stock that's orderly. In a thin one, the auction can overshoot in either direction, and market makers only step in after the open.

Placement and SPP settlement. When new shares from a capital raise land in investors' accounts, some recipients sell immediately. That selling can push the open down on settlement day for reasons that have nothing to do with the business. If the book underneath is strong, that kind of settlement-day gap can reverse very quickly once the sellers are done.

Option expiry. Expiry day can bring unusual auction volume as positions are closed or hedged, which can push opening prices around in large caps.

Fat fingers and panic. Occasionally an order is simply wrong, or a sharp overnight move triggers panic selling into the open that is out of proportion to the actual news. These get their own special topic post.

Information gaps

Information gaps are the ones you usually read about: results, guidance changes, takeover bids, regulatory approvals, contract wins, capital raises, a sharp move in the underlying commodity. The key question isn't "is the news good?" It's "is the market's reaction proportionate, and does the book confirm it?"

This was a point the journals made forcefully. Good news is not a good trade if the book can't support it. Several traders described passing on a stock with genuinely positive news because the pre-open book was thin, there were no fresh bids, and sellers arrived just before the open. The stock then faded hard. A great announcement with nothing underneath it is a trap for the people who buy the open.

The hybrid: information that has been overreacted to

Some gaps start as information gaps and become mechanical. The news is real, but the move overshoots because of panic, stop-losses, margin calls or forced selling. A small piece of bad news can produce a gap far larger than its fundamental impact. That overshoot is mechanical, even if the trigger was information.

The tell is usually proportionality: the gap is far bigger than peers, the overnight lead, or the news itself would justify.

A pre-open checklist for any gap

Here's the gap-cause checklist I distilled from the journals.

If most answers point to mechanical, the default is a fade, but only if the book offers protection. If most point to information, the default is to follow, but only if the surplus and depth confirm. If the answers are mixed, the best trade is often no trade.

Two synthetic examples

Example 1: the lone seller. A hypothetical $1.00 stock closes at 1.00. In pre-open it's indicated at 0.93. There's no announcement. The sector is flat. The book shows a single 2 million share sell order at market, and below 0.93 there are solid bids at 0.925, 0.92 and 0.915. Above 0.93, almost nothing on the offer up to 0.97. Read: mechanical. A long at the open has protection below and a clear path up as soon as the seller is filled.

Example 2: the good news trap. A hypothetical stock announces a positive update and is indicated 8% higher. Matched volume is small. The bid side under the indicative price is thin. In the final seconds, two large sell orders appear at the indicative price and the surplus flips to the offer. Read: information, but the book is not confirming. The traders I read would have pulled any long order here.

What to do with this

You don't need any special data to start. Every morning, for each stock that's gapping significantly, write one line: gap cause: information / mechanical / unclear, and one line on what the book says. Then watch what happens in the first five minutes. After a few weeks you'll have your own statistics on which kind of gap does what, in the stocks you actually trade.

The next post looks at one of the most important inputs into that judgement: the overnight lead from futures, commodities, foreign listings and sector peers.