I only know one meaning for "shooting for effect". It's the sort of thing a SWAT team commander might say to indicate that no one is terribly interested in interrogating the subjects. Basically, it means two in the chest and one in the head.
This is the first item returned by Google for the search "shooting for effect"
When shooting for effect in a combat situation, there needs to be a good balance of speed and accuracy. We need to aim small to miss small. Larger targets are easier to shoot fast because we have a smaller margin to miss, but the smaller the target gets, the harder it is to control our shots at faster speeds. We have to slow down to make sure we can make the smaller shots.
This is mostly evident when trying to transition from a body target to a head target. In the Mozambique Drill we see this the most. After two shots to the body, we transition to a smaller, harder-to-hit target (the head) for one shot. This smaller target is much harder to hit than the body shots we just took.
In competition, shooting headshots or shooting smaller steel plates is the same thing. We need to slow down after the transition and concentrate on the smaller, harder plates, especially at farther distances.
However, there seems to be some confusion about this, and this really isn't something you want to get wrong. There appears to be an alternate interpretation that shooting for effect means an ineffectual show of smoke, noise, and commotion. Basically, it's an attempt to scare someone without actually hurting them.
I was a bit surprised to find this:
It's from Reports of Cases Determined in the Supreme Court of the State of Missouri, Volume 170, apparently from a 1902 decision.
I think we would be doing everyone a service by clearing this up.
What was the first use of the phrase as it pertains to firearms?
Did it have an established legal meaning as suggested by the above court index extract?
Does it retain an established legal meaning?
When was it first used to mean shoot with deadly intent?