There is an interesting and rare concept called 'Edhiridai' Nakshatra within the parlance of Vedic Astrology. 'Edhiridai' can be translated as opposite/coming in the way/obstruction. One may have heard of 'Stambhana' or 'Vara Chakra' where a planet is said to freeze another planet. For eg. Venus is said to freeze Sun based on the weekday order. Similarly there is also a concept of Vedhaka where one planet is said to obstruct another planet and it's all the more strong when placed in a particular house from the planet concerned. For instance Venus is said to cause Vedhaka or obstruction to Sun and more so when it is said to be placed in the 11th from the Sun.
Planetary obstruction is quite common but Nakshatra obstruction is an unusual method of analysis where each planet when seated in a Nakshatra, if another planet is seated in a particular Nakshatra from the said planet then it is said to cause an impediment. Let's look at the Planets and list of Nakshatra causing hurdles to understand this concept better.
Sun - Nakshatras 5,7,8,10,12,14,15,18,21,22,23,27 - Total Count 12
Moon - Nakshatras 3,5,7,8,12,18,24 - Total Count 7
Mars - Nakshatras 3,7,10,15,21 - Total Count 5
Mercury - Nakshatras 5,11,21,22 - Total Count 4
Jupiter - Nakshatras 6,7,9 - Total Count 3
Venus - Nakshatras 20,22,24 - Total Count 3
Saturn - Nakshatras 5,6,8,10,11,20,26 - Total Count 7
Rahu - Nakshatras 20,22 - Total Count 2
Ketu - Nakshatras 20,22 - Total Count 2
Illustrating to get a grip on the understanding behind this concept. For instance let's say Sun is placed in the Nakshatra of Vishakha. If any other planet is placed in Nakshatras 5,7,8,10,12,14,15,18,21,22,23,27 from Vishakha, then that planet will cause bottleneck to the Sun. For eg. if Jupiter is placed in Revathi, it happens to be the 12th Nakshatra from Vishakha and hence becomes an obstruction to the Sun. Similarly if Saturn is placed in Shravana which is the 7th Nakshatra from Vishakha then it causes trouble to the Sun. To explain the concept for another planet Venus, if any planet placed in Nakshatras 20,22,24 from the Nakshatra in which Venus is placed, then those planets cause a barrier to Venus. It is important to note that the Nakshatra count is based on the Nakshatra in which a particular planet is placed from the listed planet's Nakshatra and not the regular order such as 1 is for Ashwini, 2 for Bharani, 27 for Revathi etc.
Let's now understand how to use this analysis in a horoscope. This works both from the stand point of Natal horoscope and also on transits/dasas. It also applies both from a Karaka Graha stand point as well as from a Bhava Adipathi view. For eg. lets say for a Gemini Lagna, 10th house Lord is Jupiter and if there is a planet in a Nakshatra placed in Edhiridai to Jupiter ie the 6th, 7th, 9th Nakshatra from where Jupiter is placed in a horoscope it can hamper work prospects for the native. This may hold good in transit as well especially when a functional malefic planet transits the 6th, 7th, 9th Nakshatra from Jupiter's placement, it can cause problems in work environment. Similarly in Dasa Analysis, if the Antar Dasa planet happens to be in Edhiridai to the Maha Dasa planet, it can cause some inconvenience. As another example we know the Karaka for mind is Moon. If any planet is placed in Nakshatras 3,5,7,8,12,18,24 from where Moon is placed, it can cause tension to the mind. Please note this is not to be confused with Tara Bala where generally Nakshatras like 8th, 24th etc are said to be favorable on account of being Mitra, Sadhak Tara respectively.
Also to be noted is that important house lords when obstructed, they get impacted more as opposed to house lords lording negative houses. For instance if the 7th Lord planet is placed in a Nakshatra that obstructs the Lagna Lord, then it can be construed that there can be problems in marital relationship as the 7th Lord of spouse opposes the Native (Lagna Lord). However if the Lagna Lord obstructs the 7th Lord then we can say that the Native is not having interest in marriage related matters. However if a planet is lording the 6th house say, it may not be such a bad thing if opposed as majority of 6th house significations already ends up becoming negative. One more thing of interest is that Sun is the most impacted planet having 12 Nakshatras as Edhiridai, incidentally even in Tithi Shoonya concept Sun owned rasi Leo or Simha is the one bearing the brunt on 5 occasions for Tithi Shoonya.
This in brief is about the concept of Edhiridai Nakshatra. If a planet is placed in specific Nakshatras from where another planet is placed, it will cause obstruction. For eg. say Mercury is placed in Arudra, the 5th star from it is Magha and if there is another planet placed in this Nakshatra will cause trouble to Mercury. Ideally, no planet should be placed in the respective star positions from the specific planet mentioned. For eg if Jupiter is placed in Ashwini, no planet should fall in the 6th, 7th or 9th star from it which is Arudra, Punarvasu & Ashlesha. This technique can be pretty helpful when scrutinizing different facets in horoscope analysis.
P.S: Picture of Nakshatras taken from internet