You saw it in extreme close up. William took the ring and, trying to place it on a finger swollen by heat, nerves and adrenalin, rucked it up at the knuckle and had to force it on. Probably painful for Kate.
The requirement at this point is that the groom take the ring and place it on his bride's finger. Holding it in place he repeats the vows.
So, at the rehearsal, good minsters will explain that there is no need to break the bride's knuckle or draw blood at this stage. Simply push the ring on as far at it will go and hold it in that position to make the vows. Then, once the vows are complete, and before the bride does the same for the groom with a hint of vengeance in her eye, invite her to make her own ring comfortable before proceeding. It's easy, straightforward and I'm amazed no-one trained our Archbishop to do it.
Thanks Ian Russell of St Jude's Mapperley. You trained me. I remembered.