In Hitman 2 you can slow time when shooting, giving Agent 47 a few seconds to line up a shot on his target. However, not all weapons have this functionality and this feature will only be available for Marksman Weapons. To see which weapons are Marksman weapons you’ll need to look for an icon on the weapon in your inventory screen. The icon that you are looking for has a white reticle and a stopwatch.
Once you’ve got a weapon equipped with this perk, any time you are shooting you can go into this slow-motion mode. To do so, you’ll need to only slightly squeeze the R2/RT button when shooting. Pressing it slowly and about half way will slow down time, allowing to aim with more precision before you take a shot. Once you have the shot lined up you can press R2/RT again to shoot. You only have a limited amount of time to take the shot before you will need to reset and do it again.
Weapons with the Marksman Perk are fairly rare in Hitman 2. You can unlock them by doing Escalation Challenges, Mastery Challenges, and completing other challenges in the game. Check out this list of Weapon Unlocks in Hitman 2 for more information on how to get them.