This is the "bowjack"
You can use it to pry boards together like this:
They are made by Vaughan Tools (maker of my beloved S2 framing hammer!) and you can get them from Lee Valley for $37.50.
I bought one two years ago and since then have put it through regular, heavy use on the construction job site. Given that there's many of these types of devices out there, I thought I would publish a short review.
So, what's the good?
First off, it does actually work pretty awesome. I've used it on standard decking, T&G ceiling, siding, Hardwood flooring, T&G decking and several other things I've forgotten. Over all, I'd say it has about a 90% success rate at working perfectly. Every time I've used it around someone who hasn't seen me use it before the reaction is the same: "I've got to get me one of those!"
The use is very easy. Whether you're laying onto a subfloor or a joist, when you run into a board that is warped or just won't go in tight to the previous board, you just grab the jack, flick it open, hammer the point into the floor or joist immediately in front of the board that you want to push in, and then pull the handle and watch the board pry in nice and tight. While hold the handle tight with one hand, you grab the screw gun or nail gun with the other and fasten the board. For long boards, obviously, you will have to start at one end and work down the line, jacking at several points, but it's all rather quick and easy.
The standard quick-and dirty method of doing this kind of thing is, of course, to grab a slot screwdriver or small chisel out of your pouch, hammer that into the floor directly in front of the board you need to pry, and then pull back on the screwdriver, thus levering the board into place. I still use that method sometimes, like in the last couple rows against a wall when there is not enough room to fit the bowjack in, but overall this bowjack is definitely a superior way to do it. For one, there's not danger of bending your screwdriver! For two, you get much greater prying power with the bow-jack. For three, the nice flat plunger of the bowjack doesn't damage the tongue or edge-of-the-board that you are prying against like the shaft of a screwdriver can.
At $37.50, it's money well spent.
After two years use, I can offer a few points of improvement.
First, the rubber handle is not very good. It's comfortable enough, but it has a bad habit of sliding off. I eventually glued mine on with clear goop glue!
Second, the tool itself is made of some sort of treated metal that doesn't rust or corrode. That's great, since tools on jobsites have a habbit of getting wet. What I noticed, however, is that the rivets used to connect the tool together at several points are not similarly protected. I've often noticed rust on them. Obviously that could become a terrible weak spot, so I've had to get in the habit of inspecting the tool regularly, and steel brushing the rust off the rivets and connectors and then giving them a shot of WD 40, which seems to repel the rust quite well for about 2 months.
Third, The point that you hammer into the joist or subfloor works well enough, but the other end, the part you're supposed to hit with your hammer, is not as well designed. As you can see in the picture, it's just a flat square of steel about 1/8" thick and about 1 1/2" wide. It can be hard to hit effectively. Sometimes the hammer just slides off of it; it's easy to make a glancing blow. If they could re-design that part to make it a little easier to hit, that would be good.
Fourth, in the second picture above you see it does have this little sliding choke-lock. The idea is that you pull the handle forward and pry the board into place, then slide this little lock over and it is supposed to keep the jack locked in place, allowing you to use two hands to fasten the board. I have to tell you, this little lock does not work at all! Not at all. Most them time it does not choke in the slightest. Sometimes if I work it a bit I can get it to hold a little, bit it immediately gives and moves a bit, about a 1/4", and thus lets the board pop back out of place. I used to fiddle with this lock all the time, thinking I was maybe just doing it wrong, but now I'm convinced it's just useless. They should either find a way to make the lock work (since it would be handy) or get rid of it.
Finally, there is a little hook on one end which I assume is for hanging it up. I wish that hook was better. The jack is too big and awkward to fit in your tool pouch, but it's the kind of thing you'd really like to keep on you -- as you move across the floor, it seems like when you need the jack it's always at the other end of the room! The best I could figure to do is to use the hook to hang the jack on my hammer loop. This doesn't work too well though, as the hook is too open, and so if I move around much or kneel down, the jack tends to fall off. Not a good thing, especially if you're working on a pre-finished floor! Maybe some sort of integrated clip or something, or else just shape the hook a little differently so that it could hang more securely. I don't know, I just wish there was a better way to keep the bowjack on you.
That's just some ways I think it could be improved. Bottom line, I think it's an awesome tool, good for so many different purposes. I keep it in my main tool bag and bring it to every job, because it really is one of those "just-in-case" tools.