F12, but aside from particle effects, I can think of only one way to do something like this and it'd still probably look like crap. Assuming that your explosion happens on the ground, half of it is above and half below. Make an explosion gfx that's just that, half of the normal. The problem is making it so that the cut-off part on bottom is aligned with the ground, and since the ground isn't always flat, this wouldn't always work.
If there's a clean way that isn't laggy to do this, F12 all the way.