Here’s my hot take, but I’m curious if anyone has more complete thoughts on this. The motor is creating motion through force. This force adds to the resistance you feel when you try to deform it. As you deform it, the motor is continuously moving the bristles back and forth. When it hits your finger you will feel that, making it feel stiffer. To bend it, you will also have to now resist the additional force of the motor trying to bring it back and force into position, increasing the stiffness you feel.
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