I've fixed up the knockback snowball.
1. Removed 'trigger', which was not used and not initialized
2. Renamed 'boost' field to 'velocity' to not shadow a local variable
3. Replace local variable 'velocity' with field; now hit() initializes field

This allows 'linger' to see the data set up in 'hit'

I've added an adjustment so the knockback is 1/10th strength; at full strength it put things in orbit!