The 1 overblow is the lowest of the over-bends, it is a semi-tone above the 1 draw and a minor 3rd above the 1 blow, therefore on a C harp it would be an Eb. It’s worth noting here that all the overblows are a semi-tone above the draw note on the same hole and all overdraws are a semi-tone above the blow note on the same hole.
With the 1 overblow you can now get a full chromatic scale in the lower register (holes 1 to 4)!
How to do it
Some folks find this a difficult one to get but I found it quite easy, so guess it’s down to the individual. The feeling comes from the back of the tongue, similar to the spot used when you pronounce ‘K’, as with all overblows the range where your tongue and mouth are in the right position is quite narrow. The shape of the mouth is similar to that of the 2 draw bend – feeling quite long, something near to the shape when making an ‘aar’ sound – and it is important to try to keep a constant pressure to sustain the note.