Wearing a pull buoy and hand paddles with swimming. Swimming with the buoy and paddles increases your stroke length from behind by providing resistance during the catch and pull phase of your stroke. Then when you swim after pulling, the goal is to maintain the feel/strength that the paddles provided without using them
Pulling with a buoy, with or without paddles also allows you to focus on your form, which tends to break down more frequently with less strength.
Pulling with a buoy only allows you to work on arm speed, adding the paddles allows you to work on pull strength, extension from behind