Sink-or-Swim drill

A few sessions of elastic band on ankle, love and hate!
I fully believe in what my coach asked me to do, just sometime it's too exhausted and trying to understand what's the other coach comment from the other part of the world. Result are all positive and align with the same angle! 

Swim band - little something we have forgotten...

- Band-only - Swimming with an ankle band is probably not one of the most popular swim drills, but it is one of the most effective. An ankle band can be made of an old bicycle tube tied into a loop, and it will eliminate the kick from your freestyle. Doing so forces a swimmer to find a way to catch more water, otherwise they don't move forward; hence, it is often called a sink-or-swim drill. When first starting out with the ankle band, it can be difficult to maintain a good body position. The legs sink and it takes quite a bit of energy to move up and down the pool. Over time, however, swimmers learn to hold more water with each stroke, and their body position will improve. At first swimmers increase their stroke rate to make up for inefficiencies in their catch, but focus on doing the drill more effectively by pulling more water. “

And then from RunnersWorld UK

Q. I recently saw someone swimming with an elastic band around their ankles. What gives?

A. Swimmers and triathletes are always looking for that extra edge to improve their performance and times. As the saying goes ‘every second counts’. There are myriad gizmos and techniques that can help make you stronger and faster.
Many top swimmers use resistance aids such as drag shorts or belts with small pouches with small holes
in them to gain extra resistance as they train. The elastic band, which can simply be an inner tube looped round and round the ankles until both your feet are bound together is a slightly different training aid. Apart from being slightly unnerving, the elastic band is a great way to improve your arm and core strength. It can also help improve your kicking.
By binding both legs together, you are actually working on your balance and engaging the core muscles of your abdomen to work more effectively. Most triathletes who aren’t from a swimming background have very poor balance and, as a result, their legs drop. This has two effects; firstly, it will slow you down as your body position is akin to swimming along with an anchor drag effect. This makes it easier for other swimmers
to swim over you.
Secondly, because your balance isn’t correct, you will overcompensate with your kicking, tiring you out more. And, again, this will slow you down.
When you try swimming with a band around your legs, you will find that if you are pulling too early in the stroke, your legs will sink. They will continue to sink until you cannot swim any more and you are in a vertical position. By concentrating on the stroke phase (in particular not pulling back immediately as your hand enters the water, but only when your hand is pointing towards the floor), you’ll glide through the water faster and flatter on top of the water. Swimming with a band will work your abdominals slightly differently and they will adjust to this training so when you swim without the band, your swimming is better.
Swimming with a band may also help with your kicking. A lot of triathletes try to compensate from their poor positioning by doing big leg kicks, which use a lot of energy. The better body positioning resulting from band training will help you use a smaller flutter kick, leaving your legs much fresher for the bike and run.