Why is PPG's amplitude squared in pwdb_pwa.m?

[yulinnauu]

`% For Pressure
sig.v = sim_data.output_data(domain_no_el).P(:,distance_el);
sig.fs = sim_data.output_data(domain_no_el).fs;
options.do_plot = 0; sig.ht = 1.75;
if domain_no_el == 7, options.do_plot = 0; end
[cv_inds, fid_pts, ~, ~] = PulseAnalyse10(sig, options);

......

% For PPG
sig.v = sim_data.output_data(domain_no_el).PPG(:,distance_el).^2;
sig.fs = sim_data.output_data(domain_no_el).fs;
options.do_plot = 0; sig.ht = 1.75;
[cv_inds, fid_pts, ~, ~] = PulseAnalyse10(sig, options);
`
The sig.v means the amplitude of the signal to be process in PulseAnalyse10. I wonder why PPG includes this “.^2”? If the PPG signal has some negative amplitude, is this process still correct?