Nasdaq shortsale rule says that you "cannot effect a short sale at or below the bid on a bid downtick" therefore, though you will have to work to see this in writing anywhere, there is nothing that says you cannot short
below the bid if the inside bid is an upbid. This is particularly relevant with decimalized, supersoes trading where the inside market is usually chaotic when a stock is showing momentum.
Take a stock that is selling off hard...just broke a major support level...futures just rolled over at resistance etc. and you would love to get short. Pre decimals and SS, you had these thick offer levels (everyone wants to sell) and thin bids, with the minimum 1/16 spread between them. So you can't hit the bid-which is shrinking fast-because of the uptick rule but if you offer it out with the other 20 participants at the offer, who the heck is going to take your offer? No one. Then the bid level is lost, a new low bid is established (still an unshortable down bid) while instantaneously everyone goes low offer at the old bid price that was just lost. Though to get short in this scenario.
As Sallyboy indicated, it is true that ARCA will re-adjust your offer price to the lowest price possible (usually the offer price if the stock is at the minimum spread) each time a new low bid/offer is established, but is this really useful since the stock is selling and hardly anyone is buying offers at this point? So finally you get filled and for a second you're happy, but you realize you just chased it 3/4 of a point and the fact that your offer was taken signifies the bounce/reversal and you watch a quick loss unfold. Frustrating but happened all the time in pre-decimal/SS days.
Fast forward to Oct 2001. What is the inside market now? Technically its the inside price level which is separated by a whopping penny or two from the next levels and so on. Now consider market maker exposure with the SuperSOES 0 second delay and executions against reserve volume and we hardly ever see MM's hanging around at these thin penny spread inside levels, rather the inside prices are dominated by individual ECN's with MM's quickly darting in and out.
So consider the example above, stock is selling hard broken support etc. Bid is a down bid so no ability to short at or below the bid. Last bidder pulls his bid at $50.01 and a new low bid of $50.00 is established (a whopping penny below the one before). These bids shrink fast and are gone in a blink and we're about to establish another new low bid at $49.99 etc.
But at that same instant some ECN trader is about to join the bid at $50.00 to cover an already profitable short (or go long speculating a bottom ...who cares why..) and he clicks his buy bottom to join what he thought was the current bid at $50.00. However a split second before he does the last bidder at the $50.00 price pulled and for a second the new bid was a lower one at $49.99, though its lifespan will be about a millisecond. Now, when the ECN trader hits his buy button at $50.00 at virtually the same instant, he re-establishes $50.00 as the inside bid...only now $50.00 is an upbid. Then his measly 500 share $50.00 bid gets whacked followed by all the $49.99 bids as the stock continues to tank. So how is this relevant to a DAT trader?
Well, if you felt the stock was going to continue to gain momentum on the downside, in the "old" days shorting was difficult (join the offer and get filled at the reversal). But today, you can join the offer at a penny above the bid (rather than a 1/16) and as the inside levels fly all around in chaotic fashion, you have a much better chance of getting filled and still satisfy the uptick rule as ECN passive orders temporarily become aggressive (eg matched with the offer prices etc) as the inside prices go back and forth etc.
Or, if the stock is really tanking and you want to get short, you can watch for that brief bid uptick to occur as in the above example-it will be brief-and fire off a short order. Do you price it at the price of that brief upbid? Hint: in the example above it was one 500 share buy order against presumably many sell orders so perhaps your chances of getting those shares is slim. So, what happens if you price the short order at say $49.97 (would you give up 2 cents to get short in a tanking stock?) at the brief moment the inside bid price goes to $50.00 (and an upbid) on that hypothetical ECN bid, which then immediately gets hit and the bid goes back to $49.99 (a downbid). Will your order at $49.97 stay live and go for one of the bids at $49.97 or get killed for violating the short sale rule? Remember the NAZ uptick rule above.
Answer: with SOES the order gets killed. But, with ECN's the interpretation may be different, giving you some flexibility in "effecting" short sales when downward momentum already exists. Play with it and see what you find. I hope that helps and welcome all who struggle with short execution and or understanding this post to check an example of this at:
www.tradecourse.com/modules/shorting_execution.htm