[samus]

I've always found lowering pulse to be more difficult, as calm mental states are easily disturbed by random thoughts and sense impressions, and emotions tend to persist unless replaced by something more captivating.

Raising the pulse should work by thinking of stressful situations. Or plausible scenarios that you never really want to experience. Maybe it's easier for people prone to such thinking.

[sn41]

I think you are talking of situations of maintaining a low pulse rate in normal situations. However, there are also situations where it is urgent to lower heart rates. A pulmonary specialist advised me to either do pursed lip breathing (or breathing where the exhale is much longer than inhales) or gently do a carotid massage.