I will add a bit here...
at first you wnt to be as relaxed as possible so as to decrease muscle tension; the slow heart rate is a by-product of that process. Once you ave initiated your dive reflex through the procedures you use, such as empty lung dives, full-lung dives, statics, and variables of those, your body will change your heart rate as it needs to adapt to your changed body state. This means exactly what Eric stated, and rememberthat you will be even more relaxed, despite the high heart rate during breath-ups.
For instance, simply initiating a high heart rate by any means available is not necessarily the right thing to do for freediving....you must relax for optimum benefit
Cheers from Bali,
Erik Y.
