This device consists of a heart rate monitor, a control box (ideally a laptop or something, so it could be fully programmable) and a sensory feedback system.
The heartrate monitor feeds your pulse into the control box. The control box monitors your pulse and sends a generated signal onto the feedback
system. The feedback system then feeds back the control box signals to you.
The feedback system could connect onto various of your senses, I would suggest that combinations of tactile, auditory and visual would be the easiest to implement at first.
The simplest version of the control box would just feed your pulse back to you. Other versions of it could feed back sped up or slowed down versions of your pulse, stronger or weaker versions of your pulse. You could mess with mark/space ratios and all sorts of things.
With some experimenting probably you could use this to get some really weird and probably dangerous effects.
With many of these machines attached to many people, and the feedback mechanisms all cross-wired (and possibly broadcasting to more than one person) I bet you could get some really crazy effects. (Especially if you could feed other signals into the control box.)
Perhaps it could be programmed as a meditation aid.
With this, who needs drugs?
The fundamental idea this stems from is putting in unusual feedback loops around various of your body systems.