If you called:

Heads Travel Thru; Bounce The Sides;

I would expect the Heads to Veer back-to-back, and the Sides to do nothing.

If you want the Sides to do a U-Turn Back, then you should say one of the following:

1. Heads Start Bounce The Sides;
2. Initially Centers Bounce The Sides;
3. Initially Heads Bounce The Sides.

In any case, I find this kind of choreography aesthetically unpleasing and gimmicky. I think the best thing is to drop this idea.

Also, a pet peeve of mine is that I believe callers should not omit the {anyone} dancers. That is, some callers simply call Bounce when they mean Bounce Nobody.

Ceder Chest Definition of Bounce anyone