yass simply provides a scroll function that smoothly scrolls the requested distance. By default, the various z relative scrolling, and ctrl-D,U,F,B are mapped.

By default, the function of the z and ctrl movements should not change, but they will do it smoothly. See :help yass for information on changing the function, and more detail

To control the speed of the sliding, there are two relevant options:

" default update interval in miliseconds
let g:yass_interval = 16

" default speed function
" must accept 1 argument, which is the distance remaining to travel
function g:Yass_speed(y)
  return 58 + 2*a:y + 50*tanh((a:y-10)/5)
endfunction

g:Yass_speed is called with two args, g:Yass_speed(y, speed) where y is the approximate number of lines left to travel, and speed is the speed at last calculation. The plugin handles the direction of travel by calling yass#scroll with a positive or negative value, so g:Yass_speed must be a positive valued function. The returned value of g:Yass_speed is treated as lines/second.

For more detail, see :help yass

MIT License