cheech226The less engagement you have, the more likely it is to slip with a hard impact to the spine. The more engagement you have, the more likely the part of the frame that bends will buckle with a hard impact. (more sideways force to the part that bends) That said, neither scenario is likely, and impacts to the spine of a knife are uncommon with normal use in the first place.
The engagement should increase somewhat with time and use, but if you're worried that it might fail, just push it a bit further in when it's engaged with your thumb.