Gel can only be cured by activating it with ultra violet light. When you put it under a uv light it activates the photo initiators which is what makes it cure. As they start to react the photo initiators are aggitated and move freely within the gel creating a friction heat or "heat spike". As the gel hardens they are unable to move about freely and so it settles down. Not everyone reacts and it normally passes very quickly. You MUST warn your client before they put their hand in the light that this could happen and if it does that they must remove their hand from the light box immediately.
Things that can cause a more intense heat spike - gel applied too thickly (this is the mai one), clients with very porous nails, clients with very thin or badly damaged nails, time of the month. It is also worse in summer than winter as we unable to tolerate heat as much - think about a shower, you love scorching hot showers in the winter, but would not be able to tolerate one un the summer.
You can almost eliminate it by getting to know your gel. LCN would take about 12-15 seconds for a reaction, so i would get a client to take thier hand out after 10 secs so that as the photo initiators started to react they calmed down before the spike had a chance to get a grip. I find Brisa reacts very quickly, within about 3-5 secs, so I am trying to work out a way to overcome this!
Another way is to apply with 2 thinner layers.
But to be honest it's over so quickly it's not worth worrying about that much.
HTH