And I want context. Is this a script? A save with a machine that detects stuff?

OMG sorry. I jumped here from the forum index, didn't see that it was in the scripting section. A proper answer is coming up.

if sim.parts()() then -- note the double function call
    -- stuff done when there are particles on the screen
end

-- * I couldn't find a way to query the max positions (152 and 95 below) in a portable way
-- * sim.XRES and sim.YRES give us the screen size,
--   but we would need something like sim.CELL to get the pressure map size
-- * you could have a loop at the beginning
--   that tries to read pressure from bigger and bigger coords until it errors
local highest_pressure = -math.huge
for ix = 0, 152 do
    for iy = 0, 95 do
        highest_pressure = math.max(highest_pressure, sim.pressure(ix, iy))
    end
end