I have tried this way of uncapping and it dose work OK if the cells capping have the dry look. If the cappings look wet then they do not open fast and the wax and honey get over heated and the wax reseals over the cells.
The fastest other than the automated uncapper in my HO is a capping scratcher. but don't try to lift the cappings off just scratch them with the end of the tines into the cappings about 1/8" I will post a video of my friend Jim who uncaps this way and he can uncap quickly enough to keep up with the extraction from his 60 frame extractor.
This is a long video I found online that has uncapping with an electric knife, an electric plane and a bread knife, Starting at 6:48 of the video. There are other interesting things explained in the video also, fume boards for removing honey, winter super storage, dealing with plugged out brood supers.
Here is a video from Don the Fat Beeman on some of the uncapping kinves and his opinion on them.