What's actually happening
Mercury's orbital period is 88 days, but its apparent motion through the zodiac as seen from Earth is uneven. During direct motion, Mercury spends roughly two to four weeks in each sign; when retrograde, it can remain in a single sign for considerably longer. An ingress into Aries places Mercury in the first sign of the tropical zodiac, ruled by Mars.
What the tradition makes of it
Mercury is the body of thought, language, and the transmission of meaning. In Aries, its natural disposition toward precision and analysis yields to something faster and more assertive. The thinking becomes immediate: first impression, first word, first move. The virtue of this is courage and directness. The liability is speaking before the thought has found its full shape.
The tradition describes Mercury in Aries as the mind at its most decisive and its most impatient. Arguments are won quickly or abandoned. Consensus is not the point; the point is clarity, arrived at by motion rather than by deliberation.
How to actually use it
Say the thing directly. This is good weather for the honest word, the decisive message, the communication that has been overworked into obscurity. Mercury in Aries cuts to it.
The practice is to deliver the direct message — and then to remain present for the response rather than moving immediately to the next one. Speed without listening is just noise.
When in doubt
What is the simplest version of what you are trying to say?