What is it and where is it?

[Norfolknick]

The trim tab works much like that of a powerboats - it essentially allows you to carry more unbalanced thrust (and therefore more sail) without too much leeway.

It has an obvious problem of drag though and is now a very outdated idea (it was big on the 12's during the 80s). If I understand the physics correctly (and I'm a dumb Seaman Officer so I'm probably wrong) then because the trim tab is affecting the flow nearer the fulcrum point it can achieve a greater effect from a smaller movement (to achieve the same at the rudder you would need a lot more either angle or size - and thus weather helm).