Commentary for Berakhot 68:4

I SAW ABAYE AND RAVA, WHO WOULD LEAN [THEIR HEADS]. See the Gemara in M’giloh 22b, where it says that a person, who is well known for his piety, should not fall on his face while reciting תחנון. The Gemara there relates that Abaye and Rovo avoided this problem by not falling directly on their faces. דמצלו אצלויי means that they leaned on their sides in order to avoid the ban for an honored person to fall on his face while reciting תחנון.

When prostrating, not spreading out the hands and legs.