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You compress the spring with a spring compressor and use the special tool like so. 
The top hat will be reused on the DDC coilover.
Here is the stock strut compared to the KW DDC part. Note how the KW part is shorter so the car can be lower without losing bump travel. 
The KW DDC strut is reassembled using the OEM top hat and the KW bump stop and dust shield. 
The new DDC strut assembly is placed into the car. 
