A holdfast is an iron bar in the shape of a number seven.
The long leg is dropped into the bench dog hole and tightened onto the work piece by a hammer blow.
Another knock, on the back of the holdfast head will release it.
Simple and quick. Holdfasts were in use in Roman times.
Auriou have made a traditional holdfast based on the above principles.
It is designed for 3/4" round bench holes and has a forged head (not cast steel, a chamfered arm with slightly convex pad (to stop the edge of the pad digging in to your workpiece) and a slightly undersized 3/4"steel post. The holdfast can also be used on 3/4" holes in the workbench legs.
The long arm is allows good access to work from all sides. The 13" hold-down post easily allows you to clamp deep pieces up to 5" thick.
The grip on the workpiece is extremely firm allowing you to plane, saw or chisel your work piece. As long as you have your mallet or hammer to hand this is a very quick and efficient workbench accessory.
If your worktop is very thin (1 1/2" - 2") then you may have to roughen up the steel post to add further grip to the post. For workbenches of 2" plus this should not be required as the posts are shot blasted to give additional grip.