The method of pumping for length requires the penis to make contact with the walls of the cylinder thereby shifting the vacuum to the space above the glans to create a more powerful “pull”. Â The sleeve basically adds girth to the shaft of the penis so that it can make contact with the walls of an otherwise slightly wide cylinder.
The fabric sheath may be very well suited to air pumping and add some particular qualities to the session whereas water pumping would probably be bettered served by using the silicone sleeve. However this would need to be tested to know which is more effective or at least preferred in either case.
Do let us know if you get a chance to test and find out!