Learn Swedish: Jag skrattade till när hon ryckte till. - I let out a short laugh when she flinched.

The particle till changes the verb to indicate that the action is sudden, short, and often involuntary. While skratta just means "to laugh," skratta till means "to let out a short laugh" or "to chuckle suddenly." Similarly, rycka usually means "to pull" or "to yank," but rycka till means "to flinch" or "to jerk suddenly."

Is till acting as the preposition "to" here, like laughing "to" someone?

No. When used like this, till is an aspectual particle, not a preposition of direction or recipient. It doesn't tell us where or to whom the action is directed; instead, it describes how the action happens (suddenly and briefly).

Which part of the phrases skrattade till and ryckte till should I stress when speaking?

You should stress the particle till. In Swedish phrasal verbs, the stress almost always falls heavily on the particle rather than the verb itself.

Both verbs are in the past tense, so why does one end in -de (skrattade) and the other in -te (ryckte)?