When. Once seated and settled — on the mount, in the car, in the aircraft seat — and before moving off. Not while packing and not once the journey is underway. In the fuller transmitted form it is preceded by Allahu akbar three times, so the words begin after the traveller has already stopped fidgeting.
The words. These are two verses of the Qur'an, from Surah az-Zukhruf (43:13-14), lifted into the traveller's mouth. Sakhkhara means to subject, to harness, to put a thing at another's service against its own scale. Muqrinin comes from a root meaning to be a match for, to be paired with as an equal — so ma kunna lahu muqrinin is not merely "we could not have done it" but "we were never its equal." The animal is stronger than the rider; the machine is not something a body could have overpowered. What is acknowledged is a mismatch, not an inconvenience.
The last word. Lamunqalibun is from qalaba, to turn a thing over. An inqilab is a reversal, an overturning. So the journey's real end is not the destination the ticket names but the turning-back to the Lord, and the sentence quietly reclassifies every trip as a rehearsal of a larger one.
The homeward half. A separate supplication is said on the return — ayibuna ta'ibuna 'abiduna li-Rabbina hamidun — and the two frame the trip. The outbound words say who owns the road; the returning words say who comes back.
A note on scope. Nothing in it names distance. A traveller crossing continents and a person starting the car for the school run say the same verses.