You may have noticed in Minecraft 1.8+ that mobs have this very floaty quality about them when hit. This is due to a change by Mojang where the client no longer reacts immediately to movement, and only does so when movement is received from the server (internal or otherwise). PhysicsFix is a small tweak which reverts the mob physics to behave more like Minecraft 1.7.10 and below.
Note: This mod can inadvertently make a client-side bug become visible, where sometimes a mob will appear to be partially in the ground. You may only experience this in certain situations, and it doesn't really affect gameplay. If I discover the source, I'll patch it as well.