The env_shake entity creates a shake effect that jostles the player's view around. Used to emphasize explosions, quakes, large footsteps and the like.
Name (targetname) - Name is a general attribute used to identify entities.
Amplitude 0-16 (amplitude) - Sets the general power of the shake. Scales from 0 (no shake) to 16 (very powerful shake).
Effect radius (radius) - Sets the radius around the env_shake entity that the effect will be felt. Note that this is purely for whether or not there is a shake; distance will not effect the power of the shake in any way, and if the player is outside of this radius, no shake will occur at all.
Duration (seconds) (duration) - Sets how long the shake lasts. The effect will gradually fade out towards the end of the shake.
0.1 = jerk, 255.0 = rumble (frequency) - Sets how erratic the shake will be. Scales from 0.1 (a mere jolt) to 255 (an earthquake-like rumble).
GlobalShake (1) - If enabled, the shake will occur everywhere regardless of the Effect Radius attribute.
If you're not stepping on floor while it triggers it won't cause any effect on your screen.