Acts as a ribosome collision sensor. Detects stalled/collided disomes (pairs of ribosomes where the leading ribosome is stalled and a second ribosome has collided with it) and endonucleolytically cleaves mRNA at the 5' boundary of the stalled ribosome. Stalled/collided disomes form a new interface (primarily via the 30S subunits) that binds SmrB. Cleaved mRNA becomes available for tmRNA ligation, leading to ribosomal subunit dissociation and rescue of stalled ribosomes