local walkSpeed = 40 -- Change this number to your preferred speed local jumpPower = 50 -- Access the local player's character and humanoid local player = game.Players.LocalPlayer local character = player.Character or player.CharacterAdded:Wait()

To modify your existing script so that the player's walk speed and jump power are only activated while the Shift key is pressed, you'll want to incorporate input handling using the `UserInputService`. Here's an updated version of your script that implements this functionality:

```lua
local UserInputService = game:GetService("UserInputService")

local walkSpeed = 40  -- Change this number to your preferred speed
local jumpPower = 50

-- Access the local player's character and humanoid
local player = game.Players.LocalPlayer
local character = player.Character or player.CharacterAdded:Wait()
local humanoid = character:WaitForChild("Humanoid")

-- Function to update humanoid properties
local function updateHumanoidProperties()
    humanoid.WalkSpeed = walkSpeed
    humanoid.JumpPower = jumpPower
end

-- Set the initial properties
updateHumanoidProperties()

-- Optional: Ensure walk speed persists after respawn
player.CharacterAdded:Connect(function(char)
    character = char
    humanoid = character:WaitForChild("Humanoid")
    updateHumanoidProperties()
end)

local shifting = false

local function onInputBegan(input, gameProcessedEvent)
    if input.UserInputType == Enum.UserInputType.Keyboard and input.KeyCode == Enum.KeyCode.LeftShift then
        shifting = true
        updateHumanoidProperties() -- Apply the properties when shift is pressed
    end
end

local function onInputEnded(input, gameProcessedEvent)
    if input.UserInputType == Enum.UserInputType.Keyboard and input.KeyCode == Enum.KeyCode.LeftShift then
        shifting = false
        humanoid.WalkSpeed = 16 -- Reset to default walk speed
        humanoid.JumpPower = 50  -- Reset to default jump power (change as needed)
    end
end

-- Connect input events
UserInputService.InputBegan:Connect(onInputBegan)
UserInputService.InputEnded:Connect(onInputEnded)
```

### Explanation:

1. **UserInputService**: This service allows you to handle user input events like keyboard presses.
2. **Shift Key Handling**: The script listens for when the Shift key is pressed (`InputBegan`) and released (`InputEnded`).
3. **Updating Character Properties**: The `updateHumanoidProperties` function sets the WalkSpeed and JumpPower. It's called to apply settings when the character spawns and whenever the Shift key is pressed.
4. **Resetting Properties**: When the Shift key is released, the script resets the properties back to the default values (In this case, walk speed is set to 16, which is Roblox's default). You can adjust this value as needed.

Make sure to test the script in a local environment to ensure proper input handling and functionality.