I fly frequently also and always keep my seat belt loosely fastened
I suggest the term "loosely" fastened was coined by cabin crew when making the initial welcoming PA in order to play down the fact that turbulence can hurt you if you are not properly strapped in. They want to avoid scaring passengers with warnings of the inherent danger of flight. Severe turbulence against a loosely fastened seat belt can damage your internal organs due to the rebound effect not present with a firmly tightened seat belt.
Re "sudden" encounters with turbulence. if it happens in cloud it is sometimes caused by incorrect use of the airborne weather radar. The aircraft flies into the top of a storm cell unseen on radar simply because the crew were paying insufficient attention to the tilt or gain setting on their radar. I have observed crews reading a newspaper on the flight deck while at night in IMC and no one evaluating storm cells in the distance until a huge jolt throws unsuspecting occupants in the cabin off their feet. The captain expressed surprise at the sudden appearance of a storm by saying where did that bastard come from when blind Freddy ccould have seen the tilt control was way to high to pick up nearby storm tops. I always first suspect crew incompetency when someone gets hurt by "unexpected" turbulence.