When Rune procs you want to switch to rake as a CP generator until you can't put up one that is at least as good as the one already on the target.
For example, if you're playing DoC: Rune proc -> Rake -> PS is expiring so you HT -> Rake -> Rake -> Switch back to shred/mangle. This is because getting the strongest possible 15 seconds worth of Rake ticks outweighs the small amount of additional damage you get from the direct damage. The goal during the proc is to get a Rip up with a massive mastery bonus use Rake aslong as you won't nerf the bleed also Thrash can be a good use as long as it won't fuck up your chances to get a Rip.