Bosch router mounted under a table? Get this.
Some Bosch routers have a little twist knob to raise and lower the bit. It isn't the easiest thing to turn if you have your router mounted under a router table.

This little device snaps into that knob and makes it quicker and easier to adjust. Just pull it off when you use the router freehand as it isn't an advantage in that mode.

A cheap little thing that makes life in the garage easier.

Much better
What review Feeser said. All true. Hope it is available again for those that don't have one. Just a whole lot better under a router table, not really necessary if you use your router freehand only.

Expands your adjustment horizons
This knob is as much an extension of your existing adjusting knob as it is an increase in the overall size of the turning head. It snaps into your existing knob. Simply put, it makes it easier to make depth adjustments. First of all it is easier to find when under the table, and secondly it makes adjusting a little easier with such a large knob.
Not much else to say other than that. Highly recommended for your Bosch 1617 or 1618 series router.