O.K. here goes, something I might be able to answer correctly let down only by my poor description.
Single clutch sequential gearboxes have a barrel which is used to move the selector forks which engage the gears. The barrel can be slightly bigger than a smartie tube up to the size of a baton (like the relay runners use), The barrel runs parellel to the gears and has grooves around the circumference, one groove for each selector fork.
The the tip of the fork sits in this groove. (on a dinner fork this would be the bit where your hand goes)
Selector forks each engage one of two gears (so one fork for 1st & 2nd one for 3rd and 4th etc.) the groove is a straight track exept for an S shape at one point which moves the selector fork right and then left to engage each gear in turn (the S is at a diferent place on each track so the gears don't all engage at the same time).
At the end of the barrel is a ratchet which operates in both directions and means that the barrel need only be turned 1/6th or 1/8th of a turn for instance (depending on number of gears) clock or anti-clock. this is acheived by a lever on the outside of the gearbox. After this rotation the barrel remains in place while the lever which turns it snaps back to the start position. This lever can then be joined to the gear lever or paddles via a tube, cables, solenoids and wires, pneumatics or by a stale baguette and some self tapping screws.
Is that what you meant?