fasten, halt, stop
畄 centers on stopping or holding something in place: fastening, halting, or bringing to a stop.
畄 is a variant or simplified form of 留, which historically combines 田 (field) with a phonetic element. The exact development of the upper component in 畄 is uncertain.
The top part looks like a stopper or plug, and the bottom is a field 田. Imagine a plug in a field stopping the flow of water, halting everything.
For リュウ, picture a river (リュウ sounds like 'ryuu', close to 'river') being halted by a dam: the river's flow is stopped, just like 畄 means to halt.