diff --git a/docs/design/reward_function.md b/docs/design/reward_function.md
index a72fb33..9cf178f 100644
--- a/docs/design/reward_function.md
+++ b/docs/design/reward_function.md
@@ -8,6 +8,8 @@ inputs must be distributed fairly to guarantee an objective comparison between d
 - The reward function is centered around minimizing the distance to the goal or maximizing the movement towards the goal
   within a finite number of timesteps $T$.
 - To motivate efficient movement, the amount of timesteps taken to reach the goal will be used as penalty.
+- An extra penalty based on movement relative to the current step and
+the previous is used to penalize a movement away from the target.
 
 ## From reward to PPO