Training can sometimes be as simple as telling someone something. Passing information from one person to another, so that both people know the same rules. It could also be a toolbox talk, a training meeting, an online course, a training video or site demonstration.
There are many ways to get messages across, and if safer working practices are introduced, the people carrying out the work should be trained or informed before they begin the task.
If you had a contract to paint 500 houses blue and all the painters were issued with litres of blue paint, then the architect changed the colour to white, it would be a high priority to get in touch with all the painters and let them know about the new colour.
The same sense of urgency should be employed if new control measures or different working practices are introduced. If a new hazard has been identified but the people carrying out the work are unaware and have an accident, liability falls firmly at the feet of the employer.