with the french link the middle link section lies flat on the tongue ,with the dr bristol the link is on 90% to the tongue and presses onto the tongue. french link kind -dr bristol severe .good question tho
The link on a Dr Bristol lies at a different angle so that it puts pressure on the tongue, it is also longer and more rectangular in shape which adds to the pressure.
I always remember the difference by thinking that the middle part of a french link is like a peanut in a shell - goes in at the middle.
Dr Bristol has a long flat middle piece with no 'peanut'
Of course the posters above are absolutely right. However you are not alone OP in not knowing the difference. I once went into a local tack shop to hire a French link bit to try on my mare. the assistant brought me a Dr Bristol and honestly didn't know the difference until I told her. She had never heard of Dr B, even though the name was inscribed on the bit!