What happened

Slack (NYSE:WORK) stock is up 61.5%!

No, it's only up 47.1%.

Or maybe Slack is up only 59%?

Actually, all these answers may be right or wrong, depending on how you look at the Slack IPO today -- and how well you understand the concept of a direct listing IPO.

Block letters I P O

Image source: Getty Images.

So what

In contrast to an "ordinary" IPO, where a company works with its underwriters to try to pick an IPO price that will (a) give it a lot of money but (b) not give it so much money that folks allocated shares in the IPO don't get a chance to make a big first-day profit, a direct listing IPO doesn't set an IPO price at all. Instead, it lets investors decide on the right price from the very first minute of trading -- and that first "buy" order of the day becomes the stock's initial price.

Now, in Slack's case, the New York Stock Exchange, where the shares are listed, did set a "reference price" -- its best guess at where Slack would begin trading today -- yesterday; $26 was the price the NYSE settled upon. Thus, when Slack shares hit their high of the day, $42, that was in fact 61.5% above the reference price. The lowest price the shares have fetched today -- $38.25 -- was 47.1% above the reference price of $26. And $41.30 -- the price Slack shares go for as I type these words at 12:50 p.m. EDT -- is 59% more than the offer price.

But it's only a 6.9% gain from Slack's opening share price of $38.50.

Now what

After NYSE set its reference price last night, IPO watchers at TheFly.com spent most of this morning revising that number, as the price at which Slack was "indicated to open" rose steadily -- $30 to $34 as of 10:20 a.m. EDT today, $32 to $34 by 10:54, $35.50 to $36.50 at 11:20, $36 to $37 nine minutes later, $37 to $38 15 minutes after that, and finally $37.50 to $38.50 in the seconds before trading actually began at noon.

Then Slack maxed out that latest pronouncement.

Where Slack shares go from here is anybody's guess, but from where I sit, Slack's momentum doesn't seem to be slackening one bit.