I suspect you haven't heard it before because it's just a poor translation. Back in the '50s, as is all too common today, manuals were sometimes translated very poorly (don't get me started on the subject). I don't know which manual you read it in, but my guess is that it's referring to a tabulator by-pass key; using it allows you to by-pass a tab stop without clearing it. You could sort of consider it to be a tab key that ignores tab stops.

I suppose there were a lot of intricate or complex circumstances in typewriter-based offices requiring tricky workarounds for which a typist would welcome a special key. Another good example is those six-key decimal tab setups for lining up the decimal point for various size numbers; on our computers the regular tab key does it all with a single key (after someone designs the tab setup, perhaps years before you went to work there). 

It's that perpetual User Interface problem: would you rather remember which of 125 keys does what you need or which combination of 6 keys?

Uwe, in using that bypass key, would you hold it down while it bypassed all the tabs you didn't want and then let it up in time to catch the one you did want? Man, talk about typing rhythm!