I do not have a “number 1” rule of safety because there are several components of safety that I would consider indespensible. But one of those big ones is absolutely communication.
When teaching tumbling, you can not assume that the student (or the instructor, depending on your point of view) knows exactly what skill you are performing each and ever time. If you are the instructor, you can not assume that just because you spotted a stundent on a standing tuck on the last repetition that they are going to throw another standing tuck. There is no such thing as spelling things out too much for your student.
This is especially true if you are switching between performing skill and performing drills. Referring back to the tuck example, I will frequently go back and forth between straight jumps, spotted tucks and spotted tuck drills. In order to make sure there is no confusion between myself and the student, I will make eye contact with them when telling them what I want them to do. I will look for them to nod back at me, and sometimes I even have them repeat it back to me so there is no way they will make the mistake of attempting a tuck when I am expecting them to be doing a jump drill. By the way, I started using this method of communication after I made the mistake of assuming my student knew to do a standing tuck when she thought she was doing a toe touch back tuck. Being directly to the side of my student, I got a very nasty kick from that breakdown in communication. Don’t let that happen to you.
Communication is even more important when dealing with running tumbling. This is because the spotter will set up where they expect the final skill is going to be executed. If you are set up for a round off series tuck and the student only throws one handspring, you will be way out of position to assist for the tuck. To avoid miscommunication, you simply have to use the same methodical communication style. Do not rush through just because you have a long line of kids you’re trying to get through the class. If you have any doubt at all, walk down to the corner and make certain that you and the student are on the same page. When you are set up 40 feet from the corner they are standing in, it is easy to mishear something or for them to misunderstand your instructions.
When it comes to stunts, you have to be even more aware of communication. This is because there are more moving parts. Make certain that everyone knows the entrance into the stunt you are using (walk in, toss, single bounce, etc). Make sure everyone knows the height of the skill you are going to0 (thigh, shoulder or extended). Make sure everyone knows what if any transitions you are executing and what counts they are going on. And the big one that people ALWAYS seem to neglect is to make sure everyone knows how you are going to dismount from the stunt (pop down, cradle, twist cradle, etc).
Besides knowing presicely what skills your stunt group is going to execute, it is also important to communicate who is going to be calling out the stunt. Is it going to be you, the instructor? Is it the back spot? Are you counting straight through the sequence or stopping when you hit the final skill, then calling out “one-two, down-up” for the cradle? The last thing you want is there to be confusion between the flyer and the bases as to how and when the dismount is going to occur. There have been more elbows to backspots eyes and noses from this communication failure than I care to think about.
My final word on communication in stunts is to remember that, just like with tumbling, it is especially important to take your time and make all of your students repeat their jobs to you when you are switching things up in a sequence and/or going back and forth between stunts and drills. It might feel like you are going to slow of wasting time by going to each athlete and getting them to verify they understand, but NOTHING wastes more time than having a stunt fall badly resulting in an injury or damage to trust and confidence.