It's a clip-on unit that slips over the original unit. That makes it bigger and bulkier then I like. I prefer a control unit that mounts closer to the bar and responses quicker. Just my personal preference.
I have two 16 Radrovers and 18 Radcity Step-thru. I use the velcro strap 1859 throttle and it fits perfectly. It has never slipped or felt too bulky after +7000 miles I put on both rovers. I can just as easily use my thumb or pad part below my thumb to apply power as needed. It doesn't get in the way reaching the gear changer or on/off button for the throttle. I added an adjustable stem to my Rovers since I have family that also ride my ebikes. Makes it real easy to adjust the angle of the thumb throttle for best fit as the handle bars go up/down. The low cost of this mod is another big plus compared to converting to thumb throttle only. You can always do the thumb trottle conversion if the 1859 still isn't to your liking.