How do I make the animation in my Shift To Crawl script play and stop instantly?

local char = script.Parent
local hum = char:WaitForChild("Humanoid")
local animator = hum:WaitForChild("Animator")
local animate = char:WaitForChild("Animate")
local ui = game:GetService("UserInputService")
local crawlin = false



ui.InputBegan:Connect(function(input)
    if input.KeyCode == Enum.KeyCode.LeftShift and crawlin == false then
        crawlin = true
        hum.WalkSpeed = 10
        hum.JumpPower = 25
        animate.walk.WalkAnim.AnimationId = "rbxassetid://10912465913" --action priority
        animate.idle.Animation1.AnimationId = "rbxassetid://10918439489" --idle priority
        animate.idle.Animation2.AnimationId = "rbxassetid://10918439489" --idle priority
    elseif  input.KeyCode == Enum.KeyCode.LeftShift and crawlin == true then
        crawlin = false
        hum.WalkSpeed = 16
        hum.JumpPower = 50
        animate.walk.WalkAnim.AnimationId = "rbxassetid://180426354" --action priority
        animate.idle.Animation1.AnimationId = "rbxassetid://180435571" --idle priority
        animate.idle.Animation2.AnimationId = "rbxassetid://180435792" --idle priority
    end
end)

While it does work, there's a slight issue. When "Shift" is pressed, the animation doesn't play instantly until the player moves. In addition, if the player is walking and pressed shift, the animation wouldn't work until they have stopped. Same thing happens when the player is crawling and presses shift. I want my crawl to look like the one in Flee The Facility on Roblox if it helps, any help is appreciated!