Its maybe not a wave but its more like a stream hitting a weir and getting deflected up sharply. the resulting flow undulates a couple of times befor dampening out.
In this case when it does happen it usually can't be really felt south of the campsies
It almost seems like a right of passage to the instructors up north. The old hands who have moved on warn them about what can happen. Three months later in the pub you listen to the next instructor to discover that a C172 can fly with ice hanging off the spars to past the door. And you can go up at 700ft/min while at idle and Vne. And if you poo yourself then you better wait for the follow through when it starts taking you down at 1000ft/min at max power and best rate climb speed.
And the most important lesson that flying IFR in IMC in a PA38 or C172 over the Cairngorms really is a F***ing stupid idea.