1) Reynold Xin presented on using sketches like Bloom filters, HyperLogLog, count-min sketches, and stratified sampling to summarize and analyze large datasets in Spark. 2) Sketches allow analyzing data in small space and in one pass to identify frequent items, estimate cardinality, and sample data. 3) Spark incorporates sketches to speed up exploration, feature engineering, and building faster exact algorithms for processing large datasets.