Well, when you score a crit, there is random roll on 0-100 on a table (unknown to player).
For example (following are just example, nothing to do with real crit table), 10-20 roll will be *1.5 dmg, 40-50 will be *3 damage knocked down, 90-100 will be *5 damage knocked out and blinded.
There are perks which modify this roll: man of steel do -10, better critical +20...