An event bound to Humanoid.StateChange:Connect would allow you to check if the player was falling and only then set the change in gravity
local mass = 0
local Humanoid = player.Character.Humanoid
for i,v in pairs(player.Character:GetChildren()) do
if v:IsA(“BasePart”) then
mass = mass + v:GetMass()
end
end
mass *= .70
humanoid.StateChanged:Connect(function(oldState, newState)
if newState == Enum.HumanoidStateType.Freefall then -- Not sure if it is Freefall or Freefalling
local Float2 = Instance.new(“VectorForce”)
Float2.Parent = player.Character.HumanoidRootPart
Float2.Force = Vector3.new(0,196.2,0) * mass
Float2.ApplyAtCenterOfMass = true
Float2.Attachment0 = player.Character.HumanoidRootPart.RootRigAttachment
Float2.RelativeTo =“World”
else
-- Set mass back to normal
end
end)