Decypher complex (to me) statement

[wavic]

It is called list comprehension.
And it is equivalent to:

event_arr = []
for x in _485_trace.split(" "):
    if x:
        event_arr.append(x) # the first x in the expression

[x for x in _485_trace.split(" ") if x] appends x to event_arr if there is non-empty x.
str.split method splits the strin by a blank characters so the argument " " is not necessary.