I think these are perhaps the most misunderstood throws.
Scobel Wiggins Photography
Younger/inexperienced players are often discouraged from practicing upside down throws. I do think that other throws should take priority when developing your throws, but there certainly is a time and place for upside down throws.
Developing upside down throws (scoobers, hammers, knifes, blades and thumbers) in place of more traditional fundamentals (forehand and backhand - and the various release points for both) is not a wise practice. I do think that the traditional throws will generally serve you better. But, adding weapons to the arsenal is always something I endorse.
Now, upside down throws do seem to end in turns a disproportionate amount of the time. This is so even when the throws themselves are good (both the look being good, and the actual execution). Interestingly, the fault lies with the thrower less than one might think.
Upside down throws are harder to catch. There are a couple of reasons for this. For example, the disc cuts through the air, and thus is often traveling faster. Further, the angle the disc is descending on can make the catch difficult. Lastly, and perhaps most importantly, the receiver has simply not had as much practice catching upside down discs. This is crucial, and, in my mind, is one of the principal reasons that upside down throws result in turns. The ratio of good look to turnover is disproportionate in upside down scenarios in part because 1) the throw is generally more unexpected, and 2) the throw is generally more difficult to catch.
That being said, upside down throws are very effective break throws because of where they are released (not to mention being more unexpected from a defender's perspective, meaning that the reaction time will be slower). Many players are able to effectively use little scoobers and knifes to get a quick break off. Others are able to utilize the hammer to break the field wide open. I think we can all stand to work the upside down throw into a more regular part of our offences. It's the extremes that we should shy away from. Falling in love with these throws is a real danger. So is surprising your teammate with an upside down throw - even if the look is good.
those are some of my thoughts.
ps - a tip for improving hammers: practice throwing blades. a good blade goes a long way to developing a good hammer.
pps - the double helix scoober is not for the faint of heart. frankly, i didn't even know it was possible until i saw this: