You should never drive a forklift with the mast raised. There are several reasons for this. One of the reasons is that you might accidentally impale something simply because you can’t look up and drive at the same time which means your gaze is straight ahead instead of up where the forks are.
Besides that, driving with the mast up drastically changes the center of gravity which puts you at risk of tipping the whole thing over.
Check out this video to see what we’re talking about here.