The five combinations
Jwalamukhi Yoga forms when Pratipada falls with the Mula nakshatra; Panchami with Bharani; Ashtami with Krittika; Navami with Rohini; or Dashami with Ashlesha. The tithi may be in either paksha. The yoga lasts while both the tithi and the nakshatra are in force together.
Its effects
The texts on muhurta say that work begun in Jwalamukhi Yoga "burns", as if in fire: a wedding may bring widowhood, a housewarming may bring loss, a journey danger, and a well dug may stay dry. Even a day that is otherwise good is rejected when this yoga is present.
Among the other yogas
Jwalamukhi is one of several such combinations of tithi, weekday and nakshatra described in muhurta works, alongside Dagdha, Visha, Hutashana, Mrityu and Krakacha yogas. Good combinations, such as Amrita Siddhi and Sarvartha Siddhi yogas, are sought instead. A muhurta is chosen by looking at all of these together with the tithi, nakshatra, weekday, yoga, karana and lagna.
These combinations are different from the 27 yogas of the panchang (Vishkambha to Vaidhriti), which come from the sum of the Sun's and Moon's longitudes.
How to check for it
Look at the tithi and nakshatra of the day in the panchang, and note when each ends. If one of the five pairs is in force together at any time of the day, that part of the day carries Jwalamukhi Yoga. When the tithi or nakshatra changes, the yoga ends, and the rest of the day may be usable. The muhurat and shubh dates pages of the site already leave such times out.