By partially extracting the cork it makes it easier to pull it out the rest of the way. Partial pullers all have some form of bell at the base of the shank.
Two American companies Edwin Walker & The C. T. Williamson company produced significant numbers of corkscrews with bell caps & indeed, Williamsons produced this type of corkscrew right up to the mid 1900's.
The example to the right has a small bell cap & is probably a German copy of William Bennit's American 1883 patent.
I've enjoyed compiling this corkscrew guide, I hope it can be of assistance to you. If you'd like to get in touch it would be great to hear from you. Peter@corkscrewsonline.com