Because of the need to break contacts fast enough to prevent them wearing from arc damage, 'tumblers' were developed from the original crude knife switches.
The 'tumbler' is a mechanism which on pressing the switch lever places the contacts under an increasing spring load, and is arranged so that when sufficient force exists, at some point in the stroke, the contact set suddenly 'tumbles' rapidly into the closed or open position, reducing arcing. Early mechanisms were clunky and heavily preloaded to ensure a crisp action. Modern switches have been developed in which the forces are much lower due to smaller and lighter moving parts, precision mouldings and superior plastics, but it's the same principle.
Alan
