So from testing out I think I've figured out what most of the parameters are and what they do. Wasn't exactly what I was looking for but I'll write it out anyway in case anyone wants this effect and struggles to find any info on it like I did.
Basically it gradually takes clients from their current timescale to the desired timescale (hence the name)
desired_timescale - the desired timescale, pretty self explanatory
duration_realtime_sec - the time in seconds that this change should happen over. e.g. curr timescale is 1.0, desired is 0.1, duration_realtime_sec is 5.0, it will take 5s to go from timescale of 1 to 0.1
interpolator_type - I haven't noticed any major changes from playing with this value but I'm guessing that it uses
these to decide how to interpolate between current timescale to desired timescale
start_blend_time - The delay in seconds (real time) before it starts the process of changing timescale
If you're interested in how it does this take a look at
https://mxr.alliedmods.net/hl2sdk-cs..._shared.cpp#92 and
https://mxr.alliedmods.net/hl2sdk-cs...shared.cpp#127