The straight line from the elbow to the bit with the wrists relaxed and in neutral position and fingers softly closed over the reins gives a rider very elastic but solid connection to the horse's mouth. There are two very important components we must highlight:

Wrists - Our wrists consists of several small bones. The connection between bones is very playable giving our wrists ability to perform complex movements. There are a lot of elastic tissues in between the bones. This gives the wrist properties similar to a rubber donut in side-reins. The relaxed wrist can stretch thus creating an elastic connection with the bit. When the wrist stays relaxed and quiet the horse can take contact with confidence because the bit is steady and slightly giving at the same time.
Now, if we change our wrist from the neutral position and bend it the elasticity will be limited, especially, if we bend our wrists either up or down. Try it yourself. Take a rein/rope or a belt in your right hand and keep the wrist in neutral and elbow by your side. With your left hand try to pull the rein away creating a straight line from the right elbow to your left hand. Even though your right fingers closed over the rein you can still feel the give. Bend your wrist upward or downward and try to pull now. Have you noticed how much less give is there? If a rider constantly keeps the wrists bent the horse feels hard and unyielding connection all the time. Opening fingers will not change the quality of the connection.
Elbows - The importance of unrestricted flow of force from the horse's mouth to the elbow comes from simple physics. The elbow in the rider is positioned slightly ahead of the middle line that divides rider's body fin two parts: the front and back. This location is just above the rider's center of mass. It is well known that the closer the weight you carry to the center of mass the easier it to carry. Unlike wrists the position of elbows is regulated by big muscles that attach directly to our body. All a rider has to do to resist horse's force is engage big and powerful muscles on his back such as Latissimus Dorsi and elbow will not move forward. With the straight line between the bit and the elbow all the weight the horse creates will be transmitted to the elbow. And rider can engage his/her whole torso and back muscles to create momentary resistance. With the line broken at the wrists joints all the weight the horse creates lands on your wrists. Muscles of your arms and upper back strain to hold this weight. The difference between these two methods of force transmission is like the difference between carrying a bag of potatoes like a tray in front of you in comparison to carrying it like a backpack. If you learn to transfer the force of the resistance to your big muscles on the back you may keep your wrists and elbows joints feel somewhat relaxed even when your horse leans on the bit. Another benefit of straight line between the bit and the elbow is no pulling required on the rider's part. The moment the horse stops leaning on the bit he is rewarded with yielding that happens automatically.