A Rapid pulse indicates that the movement of Qi is being increased by a Yang factor: Heat, or Wind with Heat.  A Rapid pulse has more than four beats per breath.  Five beats per breath could be called slightly Rapid, seven beats would be very Rapid.  The more Rapid the pulse, the more heat there is trapped in that organ or in the mediumship of that position. 

Feels like: Even before counting the rate, the Rapid pulse has at least some degree of urgency about it.