I kind of disagree with leash-jerking for training, especially for a small toy-sized dog. Some dogs are really put off by the aversive type training and will kind of freeze up if they think they are being disciplined when trying to learn what we want and it's particularly true of toy dogs - not so much with the bouncing Boxers and intuitive Labs/GSD's who can be quite thick-skinned. And training is "talking" - it's trying to communicate in a positive way to a dog who can't know what we want but is usually excited and trying its best to figure us out. I kind of make a game out of training to keep up the dog's interest in me, amping myself up to keep energy and fun and excitement in each command and response to the dog getting it right. Dogs LOVE that. They love to know they just got it right and you get them and how very smart they are and respond with excitement and celebration, happiness in them. Dogs love that we know how very smart they are and that they understand what we want and can do it and then we show we appreciate them and their braininess. They live for that moment in training. Keep it special and fun for them. That's when the magic happens between you. To jerk the leash on a little guy still working at trying to learn just seems a tad harsh-y to me but I'm an awful softie where dogs are concerned!!! It might work really well to get some dog's attention but Tibbe and some other toys I've trained would shut down and regress - and Yorkies can be that type touchy little dog if they think we are being a bit hard on them.
Still, if it works for your dog to help him learn, I guess jerking a leash is the thing to do as long as it doesn't affect the neck area. But be sure it doesn't send a little downer message to your dog.
I have always trained using only positive reinforcement except for the words "uh oh" once they really begin to learn a trick but get it occasionally wrong. I do like my dog to get negative feedback when it makes a mistake in order to mark the incorrect response and learn the meaning of "uh oh". But I only use "uh oh" after a dog has basically learned a trick well yet is not paying attention or beginning to get a little restive during a training session. I usually quickly end the training session once he's done it right a couple more times after I've had to use "uh oh" but I do like the dog to know "uh oh" means you didn't get it right, let's go again with focus.
If you just keep offering the trade-out with the term "leave it"(or drop it or whatever term you prefer) when he's playing with his toy and dropping it voluntarily in play, in time he'll learn there's a game on of some kind with you and then you can actually start teaching him to do it by saying "leave it" just as you see he's about to drop a toy. A well-placed "leave it" just before he starts to release the toy or chewie and an immediate treat/happy praise the minute the toy/chewie hits the floor and before long, they get it in a few days - some way sooner. Like anything some dogs learn certain tricks really fast and it takes a long time to pick up others.