Heat spikes are exothermic reactions, which literally means they're reactions that give off heat. When curing both acrylic and gel, chemically there is always some release of heat (known as “heat of polymerization”). The amount of heat that's generated is directly related to the number of bonds that are created during polymerization — so the more bonds that are formed, the more heat is generated. When the cure is slow and controlled, little heat is generated. But when the cure is fast, the heat is generated all at once, creating that painful heat spike.
To avoid this happening, first and foremost, use less gel. The more gel that's applied to the nail, the more heat is generated. Instead, use a thin amount of gel in multiple layers to avoid overloading the nail. Also, it's important to check your light. Ideally you should be using an LED light that matches your gel, as they were formulated to work together. Gelish 18G Plus with Comfort Cure LED Light goes a step further, as it's designed to gently and slowly cure gels so there aren't any heat spikes. If clients are particularly sensitive and still feel some heat, advise them to take their hand out of the light and gently press down with their fingertips. This usually alleviates any discomfort quickly.