Wool rubbing is a simple and easy way to try and encourage loose hairs to knot. I will often recommend it in conjunction with a saltwater spray as both methods are known to help get loose hairs knotting up.
If you've ever tried to pull a wool sweater off over a head of long non-dreaded hair you'll know what I'm talking about with wool. When you pull a wool garment over normal hair it will make the hair go super frizzy, knotty and even static-y and stand on end. This is really not a desirable look or effect for normal hair, but can be really useful with dreadlocks.
Take a wool item of clothing, a sweater, a hat, or in my case some gloves and just go to town on your head. I find a circular rubbing motion comes the most natural but any rubbing motion will work fine. This will get the loose hairs to start to cling together and will encourage them to knot. While they're in the wool rubbed state the loose hairs can often stick out a little, but you'll also find they're rather more workable than normal so you can pull them down and they'll stay in the direction you've pulled them, which is an added benefit.
Wool rubbing can be used with all types and ages of dreadlocks and can even be used to help start dreadlocks from scratch as it can kick start a head of non-dreaded hair down the neglect/natural route.
Something to watch is that it can start to get dreadlocks to stick and congo together because the wool rubbing does not differentiate between the hairs it makes cling together so watch out for congos forming! Personally I would not recommend using wool rubbing specifically for starting congos because you have no control over which dreads you're going to make stick together.