An old lady left a window open, and went in dark to close it. To prevent burglar from opening it, she actuated the "snib". I can get the general idea, but only vaguely.
I found a uk window glossary:
"Snib:
The snib is the mechanism by which the bolt can be held back, (put ‘on the latch’). It can either be a button with slide movement, or a push movement." Still don't get the picture.