I strongly DO NOT recommend harvesting by inserting the key in increments. It can actually make it worse as was my experience, and there is a good reason why Flow themselves do not officially endorse this. There is also a good reason why Flow do not recommend taking individual frames out to harvest off the hive… and not very keen on taking the Flow frames out at all. The system is designed to work when the Flow frames are packed tight together to keep their structural integrity.
This leaking issue has been bothering me for a number of years, so much so that I am now selling my Flow hives. This weekend I took my supers off the hives to harvest whats left, and did so with the lid off and I could see exactly what is happening when the key is turned.
Cracking open the frames is a relatively violent action on the frame structure. Opening up a brand new unused frame is very different from opening up a fully capped frame with 3kg of honey. The frame will twist to some degree within the super even if all the frames are in there. There are several factors that will have an effect on this and that is why some report problems while others don’t. How much wax or propolis is used by the bees, temperature, type of honey and viscosity, how much of the frame is full and where… etc. This sideways twisting of frames also varies depending on how much bridge comb is between the frames - the more the better because it will make the frames more sturdy and will twist less resulting in less broken cappings. When the key is only partially inserted and turned, it puts even more stress on the frame and it will twist more increasing the chance that frame will bow sideways, risking more broken caps and leaks. The key will give the frames structural integrity only when it is fully inserted to the other end. Very easy to see this when you harvest with the lid off. If leaks are small they will not be noticed, and are not too detrimental for the hive as the bees lap it up off each other and are not overwhelmed. If the leaks are more of a flood and is overwhelming the brood then it is a problem you want to avoid.
I’m lucky that I don’t have SHB here, so I cannot comment on that, however I came across posts saying that if your hive is weak and prone to a SHB infestation you really do not want to distract your bees with a big cleaning job, taking them away from defending the hive.
If you inspect the frames often, or just before harvesting to check the honey is fully capped, as you probably should to make sure the honey is ripe, then there is more chance that you break off any bridge comb at the top and bottom, and any wax sealing gaps between the frame blades as @Eva suggested and she is right. Excess handling of Flow frames should be avoided. The tagline that “you do not disturb the bees” should be more like “do not disturb the frames”. You MUST disturb the bees and inspect the hive regularly anyway.
Owners of Flow hives with bottom trays like FH2, are less likely to notice leaks because the honey ends up the tray and won’t spill out from the bottom.
My opinion and advice is there is no magic bullet and your best bet is to only stick with the manufacturer’s instructions. You need to make sure to find a way to know whether the honey is ripe for harvest with the least possible disturbance for the frames. If you are in doubt and for any reason think you want to harvest off the hive, then take off the entire super, not just a few frames. Keep in mind that harvesting honey that is not ripe will ferment and I didn’t find the windows to be a very reliable way to know the progress of your honey but you have to learn to make a judgment. A refractometer is probably an essential tool for Flow hive owners as is learning to make mead.
The system is not perfect but works best when all the ducks are lined up. Having strong unbroken cappings is paramount to harvesting without leaks. However with beekeeping there is a lot of variance and that explains why people have vastly different experiences with Flow hives.
Hope that helps.