Sandtimers can be used to help manage positive behaviours:
You can help stop conflict in it's tracks - especially with things like taking turns when two people want to play with the same toy. 'I know you both want to play, so when the sand runs out we can switch over' and turn the timers for 1 or 3 or 5 minutes.
You can help stop tantrums or meltdowns. I know things like leaving for school when they are playing with toys or watching a few minutes of cartoons while you get yourself ready can lead to meltdowns when it's time to stop. Saying 'I know you are enjoying what you're doing, so when the timer runs out we'll stop okay?'
You can use them to indicate how long until something is finished. When they are asking when dinner will be, or when you'll be finished with work to be able to play for example. You can also use it in the reverse way - as a positive thing too! Like 'we'll kick the ball outside for 10 minutes if you read quietly for 10 minutes'. It will be a happy way to transition and stop repeated questions that tend to drive mums a little crazy.
Remember, as with learning anything, it can take some time before children grasp the concept of it.
Use them repeatedly! Be consistent, be attentive. When the timer runs out do what it is you said would happen.
Be in control! The adult gets to control the timer at all times.
Teach your child how they work best - not to touch or move them once they are turned.