Some basic thoughts on Knockdowns. I recently waded through some confusion in a game with some players and wanted to ensure the same clarifications were spread widely.
First, Knockdowns require a roll against the target's DEF and then a comparison of BODY to see if you knocked them down. I've generalized it to a roll against their DEF, since you can perform Knockdowns with Athletics, Style, Weapon and half a dozen other skills. The BODY comparison, however, is an often missed step. You have to have more BODY than your opponent or you can't knock them down. Obviously skill, secrets, having four legs, and other things can modify this value (not to mention add realism). Still, you have to have a greater value. Equal means it doesn't work. Smaller means it definitely doesn't work. If you do succeed, they get knocked prone in the space they are standing, lose -5 CF, and take your BODY damage.
Second, if you fail you don't take any damage normally unless they have some ability, skill, secret, etc. that provides for this ability. Nor do foes automatically get a show back at you. Wrong game.
Third, the CF penalty is immediate and repeatable. If you knock someone down more than once in a melee round, they keep on taking the CF penalty. So, yes, its an very good tactic to knock someone down and keep knocking them down to keep them from ever going. Especially if you stagger it among two or three people versus one foe.
Fourth, you can't knock someone down that's prone or already knocked down. They have to be up, active and moving under their own power. Manhandling someone until that point is just that. Manhandling.
Fifth, if you employ a weapon or tool to make the knockdown, the weapon must be able to handle the total BODY of your foe. Otherwise it breaks or gets damaged in the knockdown attempt and you fail.
Six, even if you have a favored position of Prone, you still take the Knockdown damage and penalties. You just don't suffer the problems associated with being prone.