Free End vs Fixed End on Spring

Here's what a free end of a spring looks like:

(The end of the string is free to move loosely)

When a force is applied on one end of the slinky and the wave travels to the other end, the wave "bounces back" and move back towards the direction it came from on the same side of the slinky.
This is because of the forces acting on the slinky as it travels towards and goes through the free end of the slinky. As seen in the diagram, at the 5th dot, the net force is pulling upwards because the end of the slinky is not tied to a pole to counteract that upward force, resulting in the wave to continue the pattern on the same side of the slinky but in opposite directions.

Here's what a fixed end of a spring looks like:

(The end of the string is fixed and cannot move loosely)
When a force is applied on one end of the slinky and the wave travels to the other end, the wave "bounces back" and move back towards the direction it came from on the opposite side of the slinky.
This is because of the forces acting on the slinky as it travels towards and goes through the fixed end of the slinky. As seen in the diagram, at the 5th and 6th dot, the net force for both of the dots is neutral. However, at the 4th dot, the net force is pulling downwards. Therefore, as the wave travels through the slinky, that downward force pulls the entire wave down. As it travels back towards the direction it came from, the wave is now on the opposite side of the slinky because it was pulled down.

In class, we had a slinky that was 2/3 metal and 1/3 plastic. A force was applied to the slinky from the metal end and as the wave went through the section of the where the metal and plastic met, a smaller wave was created and continued traveling in the same direction and side of the slinky towards the other end. At the same time an even smaller wave was create but instead reflected to the other side of the slinky and when towards the direction it came from. I asked my classmates why this might be the case and multiple students responded with the idea that when the wave changes mediums from metal to plastic, the amount of energy needed to travel through the slinky is different. For example, say it takes 8 energy units to make a wave to travel through the plastic slinky. However, the wave has 10 energy units as it is traveling through the metal side of the slinky. There is enough energy for the wave to continue traveling from the metal to the plastic side of the slinky, but the leftover 2 energy units is not enough to form another wave. Those leftover energy units creates the smaller wave that does not travel through the plastic, but bounces back and travels back to its original direction instead. Also, the smaller wave travels on the other side because the section where the metal and the plastic slinky connects acts like a fixed end because there is a change in the material of the slinky. As a result, the wave is reflected to the other side.

Although I can see where the idea is coming from, I still cannot wrap my head around the idea that the leftover energy units will bounce back and not travel through the plastic. If that is the explanation, then why do the leftover energy units not just form a smaller wave and continue traveling through the plastic part of the slinky? To understand the material, I need to keep investigating with my classmates and discuss to see what we see and think is happening in the system.