Michael D. Coogan, The Old Testament: A Literary and Historical Introduction to the Hebrew Scriptures, third edition (OUP, 2014), 99.
Following the narrative of the call of Moses in chapters 3.1-4.17 is a short passage (4.24-26) oddly inconsistent with the larger plot. Moses has finally acceded to the divine command to return to Egypt and to secure the Hebrews’ release from the pharaoh, yet one night, while Moses is on the way back, Yahweh tries to kill him. The scene is reminiscent of Genesis 32.22-32 in which a divine adversary attacks Jacob at night.., and it anticipates the divine attack on the Egyptians, also at night (Ex 12.29-32).
The narrative, which has folklore motifs, was probably originally more detailed and is therefore difficult to interpret. What is clear that Moses’s wife Zipporah averts the threat by circumcising her son and touching “his feet” with his foreskin. The term “feet,” as often in the Bible, is a euphemism for the genitals (see 2 Sam 11.8 [compare 11.11]; Isa 6.2; 7.20; Ruth 3.4, 7), but whose “feet” are being touched is unclear. A likely interpretation is that of the NRSV, which specifies the pronoun “his” by translating “Moses’.” Thus, neither Moses’s son nor Moses himself has been circumcised, which may be why Yahweh attacks Moses. Zipporah takes action, circumcising her son. By touching the bloody skin to Moses’s genitals, she makes it appear that Moses too has just been circumcised, thus tricking Yahweh into leaving Moses alone.
The Amateur Exegete 