Hot Spot is among the first warm up exercises one learns in improv. It is a fun and high-energy warm up designed to get people to act quickly, spot patterns and "get out of their heads."
Hot Spot begins with the group standing in a circle. A suggestion may be given, but it is not necessary. One person will step into the center of the circle and begin singing a song, preferably a song most of the group already knows. After a few brief lines of song someone must tap out the player in the center and take their place. That player then begins singing a new song somehow inspired by the previous song. That becomes the pattern, as player after player tap into the the center of the circle to sing a song.
Players in the surrounding circle may support by singing along, clapping hands, impersonating instruments, etc. The circle also provides support by tapping in to move the exercise along and not leave the singer in the center for too long.
This game is described in more detail in the book Truth in Comedy:The Manual of Improvisation.
Several variations of Hot Spot exist. In general, they all involve the group forming a circle and one person taking the center until she is tagged out by her replacement.
Made-up Hot Spot
Like regular hot spot, except the songs are to be made up. Players in the circle are still encouraged to sing along.
Monologue Hot Spot
Instead of singing, the person taking the center begins to tell a story. Someone from the circle usually taps in before the story is over.