Strobe Frequency Calculation, how its done?
Why "hold" variable is calculated in the given way?


hold stores the intended delay between the strobe pulses, i.e. the
stroboscope period. It is computed as an affine function of the
potentiometer cursor position because... well, that's the simplest way
to make it user-adjustable. 400&nbsp;µs is the shortest period that
(according to the program author) one may reasonably want to set.


  Why "microsCurrent", "microsPrev" and all other timings are executed?


See the Blink Without
Delay Arduino
tutorial.


  How "frequency is calculated"?


From the code:

frequency = 1000000/(microsCurrent - microsPrev);


The frequency is the inverse of the period. The period is the time
elapsed between this strobe and the previous one. The numerator is a
conversion factor to get the frequency in hertz from a period in
microseconds.


  What is "hold frequency" variable?


It's the intended frequency (inverse of the intended period),
disregarding the timing jitter that makes the actual instantaneous
frequency slightly different.