Analyzing the video frame-by-frame is pretty helpful here. Look at these two frames:
The animation is fairly quick, but the sword starts on the back. It then shifts to the hand so that you can appear holding it. It would really only take a quick changing of CFrame to accomplish this. With modern methods, it’s also easier and more comfortable than ever to get this done.
Above tutorial’s good for these kinds of things (thanks for the link and mention), though I suggest taking a slightly different approach. You will need to handle the weld management manually so that you can get the best result out of this.
As characters already have attachments in their character, you can use this to your advantage. Set up two attachment points that line up with the character: one that will indicate when the sword is on the character’s back and another where they will grip it by when they want to equip it.
Place the sword in the character. Reconstruct the weld which you would normally get if you were to use the accessory method outlined above. Attach it to their back first.
--- Sample code for placing a sword in a player character.
-- @author colbert2677
local Character = game:GetService("Players"):FindFirstChild("colbert2677").Character
local FireSword = game:GetService("ServerStorage").FireSword
local newSword = FireSword:Clone()
newSword.Parent = Character
local newWeld = Instance.new("Weld")
newWeld.Part0 = newSword
newWeld.Part1 = Character.UpperTorso
newWeld.C0 = newSword.SheathedAttachment.CFrame
newWeld.C1 = Character.UpperTorso.BodyBackAttachment.CFrame
newWeld.Parent = newWeld.Part0
Cool, it’s on their back and you have the weld now. Just need to change the CFrame.
--- Sample code for equipping a sword.
-- @author colbert2677
local Character = game:GetService("Players"):FindFirstChild("colbert2677").Character
local FireSword = Character.FireSword
local FireSwordWeld = FireSword.Weld
FireSwordWeld.Part1 = Character.RightHand
FireSwordWeld.C0 = FireSword.HoldAttachment.CFrame
FireSwordWeld.C1 = Character.RightHand.RightGripAttachment.CFrame
As for when to do these actions, set up keyframe markers in your animation and use GetMarkerReachedSignal. Add a marker where the sword’s weld should be changed from the back to the hand and vice versa for unequipping.