Float shifting or floating gears, also called "slip shifting" or "dead sticking", is the process of changing gears, typically in a non-synchronous transmission, without depressing the clutch.
The most skillful drivers can shift non-synchronous transmissions without using the clutch by bringing the engine to exactly the right rpm in neutral before attempting to complete a shift.
If done improperly, it can ruin or destroy a transmission.
References
Float shifting Wikipedia(Text) CC BY-SA