You don't have to "hurry" to put the emergency brake pin back in, it takes more than a few minutes to overheat and possible damage the magnets. Also, pulling the pin does not apply the brakes at all. What it does is apply full battery power (voltage and current) to the magnets. That causes the magnets to pull themselves out against the vertical face of the hubs...you could say "at full force." The brake shoes stay where they are, off the hubs. There must be wheel rotation, forward or reverse to move the magnets and the arm they are attached to, to cause the brake shoes to move out to the drums and apply braking. After you pull the pin the trailer has to move a few inches before brakes are applied.
When when you use a compass to check brakes what you are actually checking is the wiring and that the magnets move. The braking is not checked and it doesn't tell you if a magnet has worn unevenly and will result weak braking or about other problems with the brake shoes, hubs and other brake components. I am not saying not to use the compass check, I am just saying... know what you are checking and what is not being checked.
Yes, I have my own story about an embarrassing inadvertent pulling of the emergency brake pin!