One thing I learned over the past 6 years during personal training sessions is you must stay with clients while they are doing bench press / dumbbell chest press. Client safety should be the number one priority for your job.
Why do you need a spotter
If you haven’t done an exercise before and are lifting a heavy weight, then it’s close to your one rep max, you may not be sure whether you are safe or not. So you will need to have someone to spot you.
A spotter will assist you with the weight if you miss a lift, and can take a small amount of the weight away from bar / dumbbells while lifting. If the spotter can relieve you of 20lb to 50lbs of the weight, then you should be able to return the bar to its ending position.
However, the spotter isn’t meant to help the person lift all the weights. They are there to help you to take off “some weights” in case you need some help. Don’t assume they will take all the weights to lift them for you.
Common mistake -dumbbell chest press-
Doing bench press and having a spotter is more common than dumbbell chest press.
So how do you spot the lifter? Common mistake I see is spotting from the lifter’s elbows. This is not the best choice.
When you lift the weights that are challenging, and reach the limit, then having someone push your elbows up may make you lose control. You want to lift straight up but elbows being pushed makes the angle change (inward) .
Do this instead -dumbbell chest press-
Instead the spotter should be ready to assist the lifter by their wrists to take off some weights. In this way, the lifter won’t lose balance and be able to lift straight up.