There are many national knives around the world that are guardless - such as the Japanese tantos, Scandinavian knives - like Puukkos, Mora knives etc. - they also have the reputation of being some of the sharpest blades around. These have all existed for centuries and have been in many everyday and hard use tasks.
Part of the trick to not having one's hand slip on to some of the sharpest blades in the world, is the way the knives are gripped.
I figured out the reverse/stab hold without much problem -
just put the thumb over the butt/pommel - like an "ice-pick" grip -
(disclaimer: A knife with a good substantial guard is intrinsically "safer" - but its safety is still further enhanced if this thumb over the pommel grip is used - so one is not entirely reliant on the guard alone)
Also another "safe" way of doing this is to steady the handle with one hand and hit the butt/pommel with the other - then one is not reliant on the hand grip at all.
For the normal cutting grip - especially for pushing hard with the point - I learnt with "sudden enlightenment"

when trying to dig an arrow head stuck hard in a tree, with a guardless knife - it dawned on me how easy it would be for my hand to slip on to the blade - then just as quickly it also dawned on me to "palm" the knife handle - ie: place the handle butt against the palm near the heel of the hand - like:
so when one pushes the pressure is actually against the palm, with no danger of the hand slipping on to the blade.
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Vincent
http://UnknownVincent.cjb.net/
http://UnknownVT.cjb.net/