What the fellow was doing was checking the amount of rebound damping that was set.
It's not that he was trying to depress the suspension rather he was trying to gauge the degree of rebound damping.
The suspension should quickly rebound (within approx 1 second), and stop without any further motion.
Too much rebound damping and it takes too long, too little and the suspension rebounds and then starts another downward cycle.
Think of a car with totally worn out shocks. It hit a bump in the road and becomes a hobby horse, rocking back and forth. That's worn out damping.
The pre-load adjusters only vary the amount of additional compression (pre-loading) on the springs. It does not change the spring rate. Only a different spring can do that.
For springs, you look at the difference between the amount of sag (suspension travel) the bikes attains between and unladen (without rider) and laden (with rider) state.
There are various sites that state what the unladen and laden sag should be for front and rear suspension on the bike.
Almost any spring can be adjusted to meet the laden (rider) sag numbers.
What's important is the unladen numbers when the rider is removed.
That's what tells you what direction to go with spring rates (up or down).
A too soft spring will need a bunch of pre-load adjustment to meet the laden sag number.
But when the rider load is removed the unladen sag number will be too little since the spring is still under a bunch of compression from the preload. It's still trying to extend.
A too stiff spring will also meet the laden (rider) sag number, but it won't require as much pre-load adjustment. So it won't be under as much compression as the soft spring.
When the rider load is removed, it might not extend as much due to very little pre-load needed. So then the unladen numbers are too high as the suspension doesn't extend enough.
The perfect spring is one that supports the rider load at the desired sag number, and also extends just enough without rider load to meet the unladen sag number.
The only way to attain this is to change the spring to one with a different spring rate.
Rule of thumb (give a set laden sag number):
If the unladen sag number is too small, you need a higher spring rate.
If the unladen sag number is too great, you need a softer spring rate.
Springs support the bike. Dampers control the bike.