Screen shake
The camera jolts for a few frames on hits and explosions to lend blows a sense of weight.
One of the cheapest tools in the "game feel" kit: nudge the camera a few random pixels for a handful of frames whenever something lands. The brain reads the motion as force even though nothing in the world actually moved. Vlambeer's Nuclear Throne pushed it to self-parody, and Martin Jonasson and Petri Purho's talk "Juice it or lose it" (2012) made it common knowledge.
Easy to overdo: too much shake makes aiming impossible and triggers motion sickness, which is why most games now ship an intensity slider. The amplitude should decay over time rather than cut off abruptly.