This technique is ideal for kids who are making great progress with their sleep, or ones who are usually good sleepers and are just going through a bit of a phase of needing extra reassurance.
The idea is to sit a while with them, as they are getting ready to fall asleep, then you 'pop-out' of the room for a VERY short time, just 30 seconds max.
Then return and continue to sit as they fall asleep. As you gradually increase the time you are out of the room, you are giving your toddler an opportunity to fall asleep independently....while knowing you aren't far away.
Each time you 'pop-out', say something simple like 'Mum has to pop out for a drink of water, I'll be back in a sec'. Pop out quickly then return. This is also a great time to tidy up the room, dust and put away laundry etc as they can see you are busy but still nearby.
You may want to gradually increase the time you pop out for, until you are giving your toddler several minutes of opportunity to fall asleep by themselves.
Other good lines for popping out:
I just need to check on the baby, I'll be back in a sec
Mum needs to go to the loo, I'll be back in a minute
I'll just turn the washing on and I'll be back to sit with you
You get the idea - quick and believable!
You may need to also say something like 'Please stay in bed while I pop out. If you get out, I may not be able to keep sitting with you'. Then provide lots of positive reinforcement for staying in bed e.g. 'Thanks for staying in bed while I popped out, what a good boy you are for settling down like that'.