Learn Swedish: Patienten vaknade till och ryste till. - The patient woke up suddenly and shuddered briefly.

In this sentence, till does not mean 'to'. It acts as an aspectual particle that modifies the verb to describe an action that happens suddenly, briefly, or involuntarily. It is attached to both verbs to show that both the waking and the shuddering were quick, sharp actions.

How is vaknade till different from just saying vaknade?

Vaknade simply means 'woke up'. Adding the particle makes it vaknade till, which specifically implies a sudden or brief awakening. It often describes snapping awake with a start, or coming to your senses momentarily before falling back asleep.

What nuance does till add to ryste?

While ryste (the past tense of rysa) can mean shivering continuously from a fever or a prolonged chill, ryste till narrows the action down to a single, sudden, and involuntary shudder or twitch.

Could I just say vaknade och ryste till to avoid repeating the particle?

No, you generally need to keep both. Because till is a particle tightly bound to the verb it modifies, dropping the first one would leave you with a regular vaknade (a normal waking up). You would lose the 'sudden' nuance for the first action.

When speaking this sentence aloud, where does the stress fall?