The arms are a conduit for energy.
To be stiff, rigid, tense in the arms is to close the pathway for the energy to be expressed.
The energy is issued from the back, channelled up from the ground.
Essentially it's my core to your core. When you watch someone do their Tai Chi and they are moving their arms in isolation from their body it becomes apparent that only partial, stiff force can be used.
When stiff force is used the hardness can be felt by the receiver. When energy is issued correctly the receiver feels nothing but is uprooted cleanly.