Meaning of manual hilbert transform

I am in early stage of learning hibert transform, and i sumarize the step of computing Hilbert is (CMIIW) :

1. computing FFT of signal
2. rotating freq by 90 degree
3. summing the rotated freq back to original freq
4. compute IFFT

also i found the script to calculate hilbert transform (not using hilbert build-in function in Matlab). But I don't undertand what does mean the step in line 8-12. Can anyone help me to explain it?

Thanks in advance

clear, close all,clc

data=load("sig_tremor.txt"); 
d=data(:,2)';
n=length(data(:,1));
dx=fft(d);
%positive frequencies
posF = 2:floor(n/2);
%negative frequencies
negF = floor(n/2)+2:n; 
dx(posF)=dx(posF)*2;
dx(negF)=0;
hilbertd = ifft(dx);
plot(d), hold on
plot(real(hilbertd),'ro')
%NOTE : 
%Prove that Hilbert transform does not affect real part