Great point Tomoy. Its a point that seems to go wrong a lot of times in my opinion.
How violent has your hip snap and spear need to be?
In my opinion the the body roll doesnt need a violent hipsnap, at least not at low strokerates.
Andynorway describes it pretty well. Is more a sequence of motions than a sudden everyting moves at one thing.
Take a look at Thorpes bodyroll from 0.24-0.40 sec when he comes swimming toward you.
Doesnt that roll look fluent and gradual?Swoooshhh-left, swooosssh-right-swoosh -left, shooosssh-right. The very start of the roll happens with a bit extra acceleration, but then the rest of the movement happens almost automatic.
Its more like the dampended unwinding of a rotated spring. The body is on stretch in the twisted extreme poition. The stretch is realeased during the roll, and then the body is on stretch again in the other extended position.
Stretchleft- let go, stretchright- let go, stretchleft- let go, whoooshh- right, swooshh-left, swooshh-right etc etc.
Important is to start the pull after the rotatation has ended and then accelerate and windup the spring again.