The one secret way of making sure your yogurt cultures properly in as little as 2 hours is to put the culture in with some tempered milk in a blender and blend it for 10 or so seconds. Then pour it back into the rest of the tempered milk, whisk for 10 seconds or so and then strain into jars for culturing.
Sorry, can't get the links below to work, so you will have to coipy and paste to see the pictures!
Interesting method, though I have never gone to so much trouble for it. To make yoghurt, I use a standard wire hand whisk to mix the culture with the milk.
This is an old fashioned milk churn of India;
The commonest method used at home (and at smaller shops is something like this picture;
Here, milk and culture are dropped from one container to another from a height (greater than in picture) a few times.
While looking for a picture of an Indian wooden milk churn, I came across this picture;
When we were kids, my parents always had a cow or a buffalo in their back yard/garden for milk for the family. My mum used to churn a large pot of yoghurt like this most mornings. Domestic butter was made by churnng yoghurt, not milk. Of course she did not dress like the woman in the statue LOL!