As a river descends a valley, it rarely goes in a straight line.
The water will take the easiest route. If there is a lump of rock in the way it will flow round it.
As a result, a river will tend to swing from side to side.
Up near the source of the Severn, the river is already taking a curvy path. As it cuts itself a valley, that curvy path causes the valley side slopes to interlock with each other like the teeth on a zip.
A piece of higher land sticking out into the lower land (in this case the valley) is called a spur. When they come from one side and then the other, as the river flows around them, then you’ve got interlocking spurs.
As the Severn’s valley gets deeper it goes through trees. The path zigzags around the spurs, with the river, but you can’t see the interlocking pattern. However, the virtually treeless Carding Mill Valley in the Shropshire Hills has a nice view!
So do they spur the river on? If anything they probably slow it down, as the river twists and turns around them and cannot follow a direct path.