To answer your question would require a tutorial length post. So many things come into play including Filtering Enabled settings. However, I can answer "How to make the character dash forward in Roblox" for you.
First thing you will need to do is capture the player object. There are a few ways to do this, but the easiest way to capture the player object is when they first spawn into your server world. If you create a regular 'Script' and put it somewhere in your 'Workspace', all you have to do is add:
workspace.Data.Player.Value = PlayerObject
Now when a player spawns into your world, the first line of code captures the player object and stores it in the variable named PlayerObject. The second line of code that I used, takes the player object and stores it into an 'ObjectValue' container that I added and named 'Player', and then stored in a folder which I called 'Data'. Now I can get the player object from any server sided script by using:
variablename = workspace.Data.Player.Value
This can be modified anyway you please, as long as you capture and store the player object somehow. Once you have the player object, you can adjust the player's speed by using:
variablename.Character.Humanoid.Walkspeed = 50 -- or whatever number you want
Hope this helps. Good Luck.