Other than a decidedly HEMA-themed band name, what is the “scheitelhau effect”? This is a term we have been using in our club to describe instances where techniques are dependent on reaction to threat and/or fear of injury that would come from fighting with sharp swords.
The phrase comes from our experience with scheitelhau, the master-strike counter to the guard alber, being somewhat ineffective in full gear freeplay, but more effective when going low gear. The working theory is that with all your gear on, the descending strike to the head that is scheitelhau carries no actual risk of injury, so the part of your brain that wants to win sees the opening in your opponent and thrusts up into their chest from alber rather than parry resulting in a double hit. If however, you are doing low to no-gear sparring (not recommended), the descending cut is scary and your mind is much more prone to break your low guard (alber) and come up into a parry to save being hit on the head. Voila, scheitelhau works as described! Risk changes the dynamics of the fight.
If you are working on a technique that seems to result in double hits more than you think it should, first double check the source material and make sure you are confident in your interpretation. If you are confident, ask yourself if either fencer would behave differently if they were in fear of their life through the play. If the answer is yes, you may have found another instance of the scheitelhau effect.