While exploring the USG Ishimura, you are going to find Gravity Panels. These are used to power the gravity system of the ship. Following the Necromorph infestation in Dead Space though, some of these Gravity Panels have broken and are malfunctioning.
You can spot a broken Gravity Panel in the environment by its upward current flow which denotes that the panel is still active. These broken panels are deadly to any organic body that comes near them in Dead Space.
Any person (including you) or thing that passes over them gets killed. The broken panel provides no gravitational pull so anyone that comes on top of it gets pushed toward the roof with so much force that they are split into atoms.
Hence, your weapons are not the only way to kill Necromorphs in Dead Space. You can also use the environment in order to save precious ammunition. However, you need to push them in first.
Keep in mind that luring and pushing five Necromorphs into a Gravity Panel is how you unlock the Ragdoll Check achievement in Dead Space.
How to use Gravity Panels for insta-kills
Since coming into contact with these broken Gravity Panels proves fatal, the best thing to do is use them against the Necromorphs aboard the USG Ishimura.
Any enemy that walks over or gets pushed into a Gravity Panel is going to be instantly killed. However, the Necromorphs are not going to get close to them on their own. You have to do the job yourself by first luring them in and then pushing them into a broken panel for insta-kills.
Lure a Necromorph and use the Force Gun
To lure a Necromorph towards a Gravity Panel in Dead Space, you need to get their attention first. You gain aggro by simply getting close to them or shooting them from afar.
As soon as a Necromorph has your attention and starts rushing toward you, make your way to a Gravity Panel. When the enemy is close enough, equip your Force Gun and push it into the panel from a safe distance.
Melee a Necromorph
The other way to push a Necromorph into a Gravity Panel is to use your Melee attacks. If you were not aware, meleeing knocks enemies back in Dead Space. However, there comes a fair risk of getting attacked yourself since you need to get in close.
Once a Necromorph gets into position near a Gravity Panel, start using Melee attacks to push it to its doom. Just make sure that you do not fall in yourself.