Lvl 24 char - 0 str - Wooden Truncheon - no blunted fitting - vanilla singleplayer no mods, x1 rates:
RMB attacks only (no shield blocking) - you will knock the thrall down before it dies.
LMB attacks only which do less concussive damage than RMB by design (no shield blocking) - you will kill the thrall before you knock it down.
Another example but with a crafter (success with both light and heavy attacks):
Conclusion: You won’t be able to knock down certain thalls with light attacks only when using the basic truncheon with no blunted fitting or str perks. Use heavy attacks instead when clubbing your new friends until you can get better tools (blunted javelin light attacks + shield is a great alternative).
Bonus note: There is a glitch if you hold ctrl (block) while you are using a truncheon, at some point you would start doing health damage instead of concussive damage. If that happens, you have to let go of the ctrl key and stop attacking for a moment to clear the issue.