What is it?

local dwindle = {transition=easing.inQuad; alpha=0, width=1, height=1; onComplete = display.remove}

local function track(event)
    if event.phase == 'began' then
        return false
    elseif event.phase == 'moved' then
        local object = event.target
        if object.previousX and object.previousY then
            local shadow = display.newCircle(object.previousX, object.previousY, 100)
            transition.to(shadow, dwindle)
            object:toFront()
        end
        object.previousX, object.previousY = object.x, object.y
        object.x, object.y = event.x, event.y
    else
        display.getCurrentStage():setFocus(event.target)
        transition.to(event.target, dwindle)
    end
end

Runtime:addEventListener('touch',
    function(event)
        if event.phase == 'began' then
            local blob = display.newCircle(event.x, event.y, 100)
            blob:addEventListener('touch', track)
            display.getCurrentStage():setFocus(blob, event.id)
        end
    end
)

system.activate "multitouch"